bcachefs: Fix an issue with inconsistent btree writes after unclean shutdown

bcachefs: Fix an issue with inconsistent btree writes after unclean shutdown

After unclean shutdown, btree writes may have completed on one device
and not others - and this inconsistency could lead us to writing new
bsets with a gap in our btree node in one of our replicas.

Fortunately, this is only an issue with bsets that are newer than the
most recent journal flush, and we already have a mechanism for detecting
and blacklisting those. We just need to make sure to start new btree
writes after the most recent _non_ blacklisted bset.

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