The similar smc_ib_devices spinlock has been converted to a mutex.
Protecting the smcd_dev_list by a mutex is possible as well. This
patch converts the smcd_dev_list spinlock to a mutex.

Fixes: c6ba7c9b ("net/smc: add base infrastructure for SMC-D and ISM")
Signed-off-by: NUrsula Braun <ubraun@linux.ibm.com>
Signed-off-by: NKarsten Graul <kgraul@linux.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>