bcachefs: Ensure we don't use a blacklisted journal seq

Different versions differ on the size of the blacklist range; it is
theoretically possible that we could end up with blacklisted journal
sequence numbers newer than the newest seq we find in the journal, and
pick a new start seq that's blacklisted.

Explicitly check for this in bch2_fs_journal_start().

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