SUNRPC: Make cache_req::thread_wait an unsigned long

The second parameter of wait_for_completion_interruptible_timeout()
is a jiffies value whose type is "unsigned long". Avoid an
unnecessary and potentially fraught implicit type conversion at the
wait_for_completion_interruptible_timeout() call site in
cache_wait_req().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>