shared_mutex: add asserts on unlock
shared_mutex: add asserts on unlock

Make sure the unlock and unlock_shared are called
when the lock is locked exclusively or shared, respectively.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,dreamsxin/seastar,scylladb/seastar,cloudius-systems/seastar