xfs: remove unnecessary check of the variable resblks in xfs_symlink

Since the "no-allocation" reservations has been removed, the resblks
value should be larger than zero, so remove the unnecessary check.

Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>