bcachefs: Some compression improvements

bcachefs: Some compression improvements

In __bio_map_or_bounce(), the check for if the bio is physically
contiguous is improved; it's now more readable and handles multi page
but contiguous bios.

Also when decompressing, we were doing a redundant memcpy in the case
where we were able to use vmap to map a bio contigiously.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>