fallocate: fix --dig-holes at end of files

fallocate: fix --dig-holes at end of files

I discovered that making a file sparse with "fallocate -d filename"
fails on the last block of a file, because - usually being partial -
the system call only zeroes that part instead of deallocating the
block. See man fallocate(2) - section "Deallocating file space".

The expected call is punching the whole block beyond eof, which
doesn't change the file length because of flag FALLOC_FL_KEEP_SIZE.

Signed-off-by: Karel Zak <kzak@redhat.com>