btrfs: move btrfs_bio allocation to volumes.c

volumes.c is the place that implements the storage layer using the
btrfs_bio structure, so move the bio_set and allocation helpers there
as well.

To make up for the new initialization boilerplate, merge the two
init/exit helpers in extent_io.c into a single one.

Reviewed-by: Nikolay Borisov <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Reviewed-by: Anand Jain <[email protected]>
Tested-by: Nikolay Borisov <[email protected]>
Tested-by: Johannes Thumshirn <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

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