mount_block_root is also used to mount non-block file systems, so give
it a better name.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230531125535.676098-7-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>