[infiniband] Split subnet management agent client out into ib_smc.c

[infiniband] Split subnet management agent client out into ib_smc.c

Not all Infiniband cards have embedded subnet management agents.
Split out the code that communicates with such an embedded SMA into a
separate ib_smc.c file, and have drivers call ib_smc_update()
explicitly when they suspect that the answers given by the embedded
SMA may have changed.