xfs: stop using set_blocksize

XFS has its own buffer cache for metadata that uses submit_bio, which
means that it no longer uses the block device pagecache for anything.
Create a more lightweight helper that runs the blocksize checks and
flushes dirty data and use that instead.  No more truncating the
pagecache because why would XFS care?

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>