[core access]: Use MTU Capability rather than Neighbor MTU

Access implements the same algorithm for path calculation like SA.
It walks the complete path from end to end. IBA spec requires using
MTU Capability in the calculation.

Using MTU Capability instead of Neighbor MTU affects path's mtu
in case of path from a port to itself. In such cases, Neighbor
MTU is often limited by MTUCap of peer switch port, which in
general is smaller than HCA MTUCap.

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>