A mount's node_id item operations need to be locked. For now let's
use a lock that's held for the duration of the mount. It makes
it trivial for us to use it with node_id items but we'll have work
to do if we want to opportunistically get access to other mount's
node_id items while they're still up.
Signed-off-by: Zach Brown <zab@versity.com>