ibcore: Store and restore ah_attr during LAP msg processing.

During CM LAP processing, ah_attr is reinitialized on receiving
a LAP request. First likely during CM request processing.

ah_attr might get zeroed out if LAP processing fails.
Therefore, try to create a new ah_attr for the LAP message.
If the initialization fails, continue with older ah_attr.
If the initialization passes, consider the new ah_attr by
overwriting the older one.

Linux commit:
0e225dcb7681c0a8e52fb9dc68bd8ab973de4ca2

Reviewed by: kib
Sponsored by: Mellanox Technologies // NVIDIA Networking

(cherry picked from commit bf5075e41e6a29232a1c5aef07e69b9173494187)