Syzbot reports a problem that a warning is triggered due to suspicious
use of rcu_dereference_check(). That is triggered by a call of
bch2_snapshot_tree_oldest_subvol().

The cause of the warning is that inside
bch2_snapshot_tree_oldest_subvol(), snapshot_t() is called which calls
rcu_dereference() that requires a read lock to be held. Also, the call
of bch2_snapshot_tree_next() eventually calls snapshot_t().

To fix this, call rcu_read_lock() before calling snapshot_t(). Then,
release the lock after the termination of the while loop.

Reported-by: <syzbot+f7c41a878676b72c16a6@syzkaller.appspotmail.com>
Signed-off-by: Ahmed Ehab <bottaawesome633@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>