truncate: always set mode when opening file to avoid fortify errors

truncate: always set mode when opening file to avoid fortify errors

Busybox crashes due to no mode being given when opening:
$ ./busybox truncate -s 1M foo
*** invalid open64 call: O_CREAT without mode ***: ./busybox terminated
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x7338f)[0x7f66d921338f]
/lib/x86_64-linux-gnu/libc.so.6(__fortify_fail+0x5c)[0x7f66d92aac9c]
/lib/x86_64-linux-gnu/libc.so.6(+0xeb6aa)[0x7f66d928b6aa]
./busybox[0x4899f9]
======= Memory map: ========
00400000-004d0000 r-xp 00000000 00:1a 137559                             /home/ari/busybox/busybox
006cf000-006d0000 r--p 000cf000 00:1a 137559                             /home/ari/busybox/busybox
006d0000-006d1000 rw-p 000d0000 00:1a 137559                             /home/ari/busybox/busybox
006d1000-006d4000 rw-p 00000000 00:00 0
014e7000-01508000 rw-p 00000000 00:00 0                                  [heap]
7f66d8f8a000-7f66d8fa0000 r-xp 00000000 08:07 1579008                    /lib/x86_64-linux-gnu/libgcc_s.so.1
7f66d8fa0000-7f66d919f000 ---p 00016000 08:07 1579008                    /lib/x86_64-linux-gnu/libgcc_s.so.1
7f66d919f000-7f66d91a0000 rw-p 00015000 08:07 1579008                    /lib/x86_64-linux-gnu/libgcc_s.so.1
7f66d91a0000-7f66d935b000 r-xp 00000000 08:07 1578994                    /lib/x86_64-linux-gnu/libc-2.19.so
7f66d935b000-7f66d955a000 ---p 001bb000 08:07 1578994                    /lib/x86_64-linux-gnu/libc-2.19.so
7f66d955a000-7f66d955e000 r--p 001ba000 08:07 1578994                    /lib/x86_64-linux-gnu/libc-2.19.so
7f66d955e000-7f66d9560000 rw-p 001be000 08:07 1578994                    /lib/x86_64-linux-gnu/libc-2.19.so
7f66d9560000-7f66d9565000 rw-p 00000000 00:00 0
7f66d9565000-7f66d966a000 r-xp 00000000 08:07 1579020                    /lib/x86_64-linux-gnu/libm-2.19.so
7f66d966a000-7f66d9869000 ---p 00105000 08:07 1579020                    /lib/x86_64-linux-gnu/libm-2.19.so
7f66d9869000-7f66d986a000 r--p 00104000 08:07 1579020                    /lib/x86_64-linux-gnu/libm-2.19.so
7f66d986a000-7f66d986b000 rw-p 00105000 08:07 1579020                    /lib/x86_64-linux-gnu/libm-2.19.so
7f66d986b000-7f66d988e000 r-xp 00000000 08:07 1578981                    /lib/x86_64-linux-gnu/ld-2.19.so
7f66d9a64000-7f66d9a67000 rw-p 00000000 00:00 0
7f66d9a8a000-7f66d9a8d000 rw-p 00000000 00:00 0
7f66d9a8d000-7f66d9a8e000 r--p 00022000 08:07 1578981                    /lib/x86_64-linux-gnu/ld-2.19.so
7f66d9a8e000-7f66d9a8f000 rw-p 00023000 08:07 1578981                    /lib/x86_64-linux-gnu/ld-2.19.so
7f66d9a8f000-7f66d9a90000 rw-p 00000000 00:00 0
7ffc47761000-7ffc47782000 rw-p 00000000 00:00 0                          [stack]
7ffc477ab000-7ffc477ad000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
Aborted (core dumped)
$

Fix this by simply always setting the mode, as it doesn't hurt even
when O_CREAT is not specified.

This bug is a regression introduced in fc3e40e, as xopen(), which
was originally used, would automatically set the mode.

Signed-off-by: Ari Sundholm <ari@tuxera.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
(cherry picked from commit e111a1640494fe87fc913f94fae3bb805de0fc99)