xfs: Fix missing xfs_iunlock() on error recovery path in xfs_readlink()

Commit b52a360b forgot to call xfs_iunlock() when it detected corrupted
symplink and bailed out. Fix it by jumping to 'out' instead of doing return.

CC: stable@kernel.org
CC: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Alex Elder <elder@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>