mm/filemap: Add folio_wait_bit()

Rename wait_on_page_bit() to folio_wait_bit().  We must always wait on
the folio, otherwise we won't be woken up due to the tail page hashing
to a different bucket from the head page.

This commit shrinks the kernel by 770 bytes, mostly due to moving
the page waitqueue lookup into folio_wait_bit_common().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>