ibcore: Add rdma_reject_msg() helper function.

rdma_reject_msg() returns a pointer to a string message associated with
the transport reject reason codes.

Linux commit:
77a5db13153906a7e00740b10b2730e53385c5a8

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

(cherry picked from commit e25bcf8d775c61e9bb67246545d2ee6717f4ca89)