vm: fix incorrect unlock error path in madvise_free_huge_pmd

vm: fix incorrect unlock error path in madvise_free_huge_pmd

Commit b8d3c4c3009d ("mm/huge_memory.c: don't split THP page when
MADV_FREE syscall is called") introduced this new function, but got the
error handling for when pmd_trans_huge_lock() fails wrong.  In the
failure case, the lock has not been taken, and we should not unlock on
the way out.

Cc: Minchan Kim <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>