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 XFS does not use it or care about it.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>