xfs: split inode flushing from xfs_sync_inodes_ag

In many cases we only want to sync inode metadata. Split out the inode
flushing into a separate helper to prepare factoring the inode sync code.

Based on a patch from Dave Chinner, but redone to keep the current behaviour
exactly and leave changes to the flushing logic to another patch.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>