RDMA/usnic: Lock VF with mutex instead of spinlock

Usnic VF doesn't need lock in atomic context to create QPs, so it is safe
to use mutex instead of spinlock. Such change fixes the following smatch
error.

Smatch static checker warning:

   lib/kobject.c:289 kobject_set_name_vargs()
    warn: sleeping in atomic context

Fixes: 514aee66

 ("RDMA: Globally allocate and release QP memory")
Link: https://lore.kernel.org/r/2a0e295786c127e518ebee8bb7cafcb819a625f6.1631520231.git.leonro@nvidia.com
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>