IB/usnic: Use GFP_ATOMIC under spinlock

This is called from qp_grp_and_vf_bind() and we are holding the
vf->lock so the allocation can't sleep.

Fixes: e3cf00d0 ('IB/usnic: Add Cisco VIC low-level hardware driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>