current_task appears to be x86-only, oops.

Let's just delete this check entirely:

Any developer that adds a new user without setting rq_task will get a
crash the first time they test it.  I also don't think there are
normally any important locks held here, and I can't see any other reason
why killing a server thread would bring the whole box down.

So the effort to fail gracefully here looks like overkill.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 983c6844 "SUNRPC: get rid of the request wait queue"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>