mm,btrfs: add a filemap_flush_nr helper

mm,btrfs: add a filemap_flush_nr helper

Abstract out the btrfs-specific behavior of kicking off I/O on a number
of pages on an address_space into a well-defined helper.

Note: there is no kerneldoc comment for the new function because it is
not part of the public API.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20251024080431.324236-7-hch@lst.de
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>