rgw: fix lock scope in ObjectCache::get()

rgw: fix lock scope in ObjectCache::get()

in the touch_lru() case, we promote the shared_lock to a unique_lock.
but because the unique_lock is in a nested scope, the lock drops with
its scope and we continue accessing the map without any protection

this moves the unique_lock up to function scope, where it's
constructed as unlocked with std::defer_lock. after promotion, this
lock will be held until the function returns

Fixes: https://tracker.ceph.com/issues/52800
Signed-off-by: Casey Bodley <cbodley@redhat.com>