knfsd: spawn kernel thread to probe callback channel

We want to allow gss on the callback channel, so people using krb5 can
still get the benefits of delegations.

But looking up the rpc credential can take some time in that case.  And
we shouldn't delay the response to setclientid_confirm while we wait.

It may be inefficient, but for now the simplest solution is just to
spawn a new thread as necessary for the purpose.

(Thanks to Adrian Bunk for catching a missing static here.)

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Cc: Adrian Bunk <bunk@kernel.org>