Honor return status of ib_init_ah_from_mcmember() in ibcore.

The return status of ib_init_ah_from_mcmember() is ignored by
cma_ib_mc_handler(). Honor it and return error event if ah attribute
initialization failed.

Linux commit:
6d337179f28cc50ddd7e224f677b4cda70b275fc

MFC after: 1 week
Sponsored by: Mellanox Technologies