bcachefs: Guard against journal seq overflow

Wraparound is impractical to handle since in various places we use 0 as
a sentinal value - but 64 bits (or 56, because the btree write buffer
steals a few bits) is enough for all practical purposes.

Reported-by:  <syzbot+73ed43fbe826227bd4e0@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>