bcachefs: Fix journal pins in btree write buffer

btree write buffer flush has two phases
 - in natural key order, which is more efficient but may fail
 - then in journal order

The journal order flush was assuming that keys were still correctly
ordered by journal sequence number - but due to coalescing by the
previous phase, we need an additional sort.

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