bcachefs: Ensure we always have a journal pin in interior update path

bcachefs: Ensure we always have a journal pin in interior update path

For the new nodes an interior btree update makes reachable, updates to
those nodes may be journalled after the btree update starts but before
the transactional part - where we make those nodes reachable. Those
updates need to be kept in the journal until after the btree update
completes, hence we should always get a journal pin at the start of the
interior update.

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