Staging: rdma:Delete unnecessary NULL check before calling function "kmem_cache_destroy"

The kmem_cache_destroy() function tests whether its argument is NULL
and then returns immediately.
Thus the NULL check before calling this function is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Yash Shah <yshah1@visteon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>