ibcore: Don't allocate method table, if already present.

This commit aligns the code in question with upstream Linux.

Linux commit:
2468b82d69e3a53d024f28d79ba0fdb8bf43dfbf

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

(cherry picked from commit 721b795b721b349db5e6198f8681d5992447c775)