bcachefs: Don't drop journal pins in exit path

bcachefs: Don't drop journal pins in exit path

There's no need to drop journal pins in our exit paths - the code was
trying to have everything cleaned up on any shutdown, but better to just
tweak the assertions a bit.

This fixes a bug where calling into journal reclaim in the exit path
would cass a null ptr deref.

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