move the low-level btrfs_bio code into a separate file (resent)

Christoph Hellwig
2022-09-12T14:11:39Z

[resending because I forgot to Cc the btrfs list, sorry]

Hi all,

this small series creates a new bio.c file (and a bio.h header for it)
to contain all the "storage" layer code below btrfs_submit_bio. The
amount of code sitting below btrfs_submit_bio will grow a lot with
the "consolidate btrfs checksumming, repair and bio splitting" series,
so this pure code move series triest to prepare for that by making
sure we have a neat file to add it to.