IB/opa_vnic: Use spinlock instead of mutex for stats_lock

Stats can be read from atomic context, hence make stats_lock as
a spinlock.

Fix the following trace with debug kernel.

BUG: sleeping function called from invalid context at
kernel/locking/mutex.c:238
in_atomic(): 1, irqs_disabled(): 0, pid: 6487, name: sadc
Call Trace:
 dump_stack+0x63/0x90
 ___might_sleep+0xda/0x130
 __might_sleep+0x4a/0x90
 mutex_lock+0x20/0x50
 opa_vnic_get_stats64+0x56/0x140 [opa_vnic]
 dev_get_stats+0x74/0x130
 dev_seq_printf_stats+0x37/0x120
 dev_seq_show+0x14/0x30
 seq_read+0x26d/0x3d0

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>