firmware: arm_ffa: Snapshot notifier callbacks under lock

firmware: arm_ffa: Snapshot notifier callbacks under lock

Both notification handlers currently look up a notifier callback under
notify_lock, drop the lock, and then dereference the returned
notifier entry. A concurrent unregister can delete and free that
entry in the gap, leaving the handler to dereference stale memory.

Copy the callback pointer and callback data while notify_lock is
still held and invoke the callback only after the lock is dropped.
This keeps the existing callback execution model while removing the
use-after-free window in both the framework and non-framework
notification paths.

Fixes: 285a5ea0f542 ("firmware: arm_ffa: Add support for handling framework notifications")
Link: https://patch.msgid.link/20260428-ffa_fixes-v2-10-8595ae450034@kernel.org
Signed-off-by: Sudeep Holla <sudeep.holla@kernel.org>