bcachefs: Run jset_validate in write path as well

bcachefs: Run jset_validate in write path as well

This is because we had a bug where we were writing out journal entries
with garbage last_seq, and not catching it.

Also, completely ignore jset->last_seq when JSET_NO_FLUSH is true,
because of aforementioned bug, but change the write path to set last_seq
to 0 when JSET_NO_FLUSH is true.

Minor other cleanups and comments.

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