fix cleanup in kernel code

RM issue: 399577 Null dereference: no device at
"xio_rdma_primary_pool_destroy"

Scenario: unloading xnbd module i.e. "modporobe -r xnbd".
Fix, handle xio_cq refcount and fix rwlock.

Signed-off-by: Shlomo Pongratz <shlomop@mellanox.com>