mm/gup: add an assertion that the mmap lock is locked

Since commit 5b78ed2 ("mm/pagemap: add mmap_assert_locked()
annotations to find_vma*()") we already have this assertion, it is just
buried in find_vma():

 __get_user_pages_locked()
  __get_user_pages()
  find_extend_vma()
   find_vma()

Also check it at the top of __get_user_pages_locked() as a form of
documentation.

Link: https://lkml.kernel.org/r/6-v2-987e91b59705+36b-gup_tidy_jgg@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/gup.c

@@ -1360,6 +1360,8 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
 		must_unlock = true;
 		*locked = 1;
 	}
+	else
+		mmap_assert_locked(mm);
 
 	if (flags & FOLL_PIN)
 		mm_set_has_pinned_flag(&mm->flags);