bcachefs: Fix shift-out-of-bounds in bch2_blacklist_entries_gc

This series fix the shift-out-of-bounds issue in
bch2_blacklist_entries_gc().

Instead of passing 0 to eytzinger0_first() when iterating the entries,
we explicitly check 0 and initialize i to be 0.

syzbot has tested the proposed patch and the reproducer did not trigger
any issue:

Reported-and-tested-by:  <syzbot+835d255ad6bc7f29ee12@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=835d255ad6bc7f29ee12
Signed-off-by: Pei Li <peili.dev@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>