Fix intermittency in refquota_002_pos from the ZFS test suite

tests/sys/cddl/zfs/bin/mkfile.c
Don't unlink a file when unable to write the entire thing. Nothing
in the ZFS test suite depends on that behavior. However, a few
tests (like refquota_002_pos) depend on the opposite. Right now
refquota_002_pos only passes sometimes because unlink(2) loses a
race against "zfs get used".

Sponsored by: Spectra Logic Corp