xfs: refactor live buffer invalidation for repairs

xfs: refactor live buffer invalidation for repairs

In an upcoming patch, we will need to be able to look for xfs_buf
objects caching file-based metadata blocks without needing to walk the
(possibly corrupt) structures to find all the buffers.  Repair already
has most of the code needed to scan the buffer cache, so hoist these
utility functions.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>