net/smc: use mutex instead of rwlock_t to protect buffers

The locks for sndbufs and rmbs are never used from atomic context. Using
a mutex for these locks will allow to nest locks with other mutexes.

Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Reviewed-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>