block: Introduce new bio_split()

block: Introduce new bio_split()

The new bio_split() can split arbitrary bios - it's not restricted to
single page bios, like the old bio_split() (previously renamed to
bio_pair_split()). It also has different semantics - it doesn't allocate
a struct bio_pair, leaving it up to the caller to handle completions.

Then convert the existing bio_pair_split() users to the new bio_split()
- and also nvme, which was open coding bio splitting.

(We have to take that BUG_ON() out of bio_integrity_trim() because this
bio_split() needs to use it, and there's no reason it has to be used on
bios marked as cloned; BIO_CLONED doesn't seem to have clearly
documented semantics anyways.)

Signed-off-by: Kent Overstreet <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Martin K. Petersen <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Keith Busch <[email protected]>
Cc: Vishal Verma <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Neil Brown <[email protected]>