ANDROID: GKI: bring back find_extend_vma()

In commit 8d7071af ("mm: always expand the stack with the mmap write
lock held"), find_extend_vma() was no longer being used in the tree, so
it was removed.  Unfortunately some GKI external module is using this,
so bring it back to allow things to continue to work.

Bug: 161946584
Fixes: 8d7071af ("mm: always expand the stack with the mmap write lock held")
Change-Id: I6f1fb1fd8193625fe3dac0bbc5b0aff653b3d879
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>