net/smc: protect smc ib device initialization

Before an smc ib device is used the first time for an smc link it is
lazily initialized. When there are 2 active link groups and a new ib
device is brought online then it might happen that 2 link creations run
in parallel and enter smc_ib_setup_per_ibdev(). Both allocate new send
and receive completion queues on the device, but only one set of them
keeps assigned and the other leaks.
Fix that by protecting the setup and cleanup code using a mutex.

Reviewed-by: Ursula Braun <ubraun@linux.ibm.com>
Fixes: f3c1dedd ("net/smc: separate function for link initialization")
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>