mm: move mmap_lock assert function definitions

Move mmap_lock assert function definitions up so that they can be used by
other mmap_lock routines.

Link: https://lkml.kernel.org/r/20230227173632.3292573-12-surenb@google.com


Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>