du: use long long for size field

du: use long long for size field

du has issues with files reporting large block counts on
32-bit systems with 4 byte longs.  From looking at the stat.c code, it
seems the preference is to use 'long long', rather than blkcnt_t.

function                                             old     new   delta
du                                                   420     444     +24
du_main                                              317     321      +4
print                                                 43      41      -2
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 28/-2)              Total: 26 bytes

Signed-off-by: Ian Wienand <ianw@vmware.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>