ksm: unstable_tree_search_insert error checking cleanup

ksm: unstable_tree_search_insert error checking cleanup

get_mergeable_page() can only return NULL (also in case of errors) or the
pinned mergeable page.  It can't return an error different than NULL.
This optimizes away the unnecessary error check.

Add a return after the "out:" label in the callee to make it more
readable.

Signed-off-by: Andrea Arcangeli <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Cc: Petr Holasek <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>