nfsd4: simpler ordering of setclientid_confirm checks

The cases here divide into two main categories:

	- if there's an uncomfirmed record with a matching verifier,
	  then this is a "normal", succesful case: we're either creating
	  a new client, or updating an existing one.
	- otherwise, this is a weird case: a replay, or a server reboot.

Reordering to reflect that makes the code a bit more concise and the
logic a lot easier to understand.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>