ksm: unstable_tree_search_insert error checking cleanup

ksm: unstable_tree_search_insert error checking cleanup

get_mergeable_page() can only return NULL (in case of errors) or the
pinned mergeable page.  It can't return an error different than NULL.
This makes it more readable and less confusion in addition to optimizing
the check.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Petr Holasek <pholasek@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>