block: hoist block size validation code to a separate function

Hoist the block size validation code to bdev_validate_blocksize so that
we can call it from filesystems that don't care about the bdev pagecache
manipulations of set_blocksize.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/174543795720.4139148.840349813093799165.stgit@frogsfrogsfrogs


Signed-off-by: Jens Axboe <axboe@kernel.dk>