ipoib: Fix incorrectly computed IPOIB_CM_RX_SG value.

The computed IPOIB_CM_RX_SG is too small. It doesn't account for fallback
to mbuf clusters when jumbo frames are not available and it also doesn't
account for the packet header and trailer mbuf.

This causes a memory overwrite situation when IPOIB_CM is configured.

While at it add a kernel assert to ensure the mapping array is not overwritten.

PR: 254474
MFC after: 1 week
Sponsored by: Mellanox Technologies // NVIDIA Networking