When we didn't find anything in the journal that we'd like to use, and
we're forced to use whatever we can find - that entry will have been a
JSET_NO_FLUSH entry with a garbage last_seq value, since it's not
normally used.

Initialize it to something sane, for bch2_fs_journal_start().
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>