RDMA/vmw_pvrdma: Use refcount_dec_and_test to avoid warning

refcount_dec generates a warning when the operation
causes the refcount to hit zero. Avoid this by using
refcount_dec_and_test.

Fixes: 8b10ba78

 ("RDMA/vmw_pvrdma: Add shared receive queue support")
Reviewed-by: Adit Ranadive <aditr@vmware.com>
Reviewed-by: Aditya Sarwade <asarwade@vmware.com>
Reviewed-by: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: Bryan Tan <bryantan@vmware.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>