bcachefs: Fix snapshot_t() usage in bch2_fs_quota_read_inode()

bch2_fs_quota_read_inode() wasn't entirely updated to the
bch2_snapshot_tree() helper, which takes rcu lock.

Reported-by:  <syzbot+a3a9a61224ed3b7f0010@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>