mm/filemap: Add folio_wait_locked()

Also add folio_wait_locked_killable().  Turn wait_on_page_locked() and
wait_on_page_locked_killable() into wrappers.  This eliminates a call
to compound_head() from each call-site, reducing text size by 193 bytes
for me.

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>