block/file-posix: Truncate in xfs_write_zeroes()

XFS_IOC_ZERO_RANGE does not increase the file length:
$ touch foo
$ xfs_io -c 'zero 0 65536' foo
$ stat -c "size=%s, blocks=%b" foo
size=0, blocks=128

We do want writes beyond the EOF to automatically increase the file
length, however.  This is evidenced by the fact that iotest 061 is
broken on XFS since qcow2's check implementation checks for blocks
beyond the EOF.

Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>