bcachefs: allow writeback to fill bio completely

The bcachefs folio writeback code includes a bio full check as well
as a fixed size check to determine when to split off and submit
writeback I/O. The inclusive check of the latter against the limit
means that writeback can submit slightly prematurely. This is not a
functional problem, but results in unnecessarily split I/Os and
extent merging.

This can be observed with a buffered write sized exactly to the
current maximum value (1MB) and with key_merging_disabled=1. The
latter prevents the merge from the second write such that a
subsequent check of the extent list shows a 1020k extent followed by
a contiguous 4k extent.

The purpose for the fixed size check is also undocumented and
somewhat obscure. Lift this check into a new helper that wraps the
bio check, fix the comparison logic, and add a comment to document
the purpose and how we might improve on this in the future.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>