IB/core: Add locking around event dispatching on XRC target QPs

Fix a potential race when event occurrs on a target XRC QP and in the
middle of reporting that on its shared qps, one of them is destroyed
by user space application.  Also add note for kernel consumers in
ib_verbs.h that they must not destroy the QP from within the handler.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>