bcachefs: Add an assertion in bch2_journal_pin_set()

Previously, bch2_journal_pin_set() would silently ignore a request to
pin a journal sequence number that was no longer dirty, because it was
used internally by bch2_journal_pin_copy() which could race with the src
pin being flushed.

Split these apart so that we can properly assert that @seq is a
currently dirty journal sequence number - this is almost always a bug.

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