fix remaining direct use of stat syscalls outside fstatat.c

fix remaining direct use of stat syscalls outside fstatat.c

because struct stat is no longer assumed to correspond to the
structure used by the stat-family syscalls, it's not valid to make any
of these syscalls directly using a buffer of type struct stat.

commit 9493892021eac4edf1776d945bcdd3f7a96f6978 moved all logic around
this change for stat-family functions into fstatat.c, making the
others wrappers for it. but a few other direct uses of the syscall
were overlooked. the ones in tmpnam/tempnam are harmless since the
syscalls are just used to test for file existence. however, the uses
in fchmodat and __map_file depend on getting accurate file properties,
and these functions may actually have been broken one or more mips
variants due to removal of conversion hacks from syscall_arch.h.

as a low-risk fix, simply use struct kstat in place of struct stat in
the affected places.