nfsd: Fix message level for normal termination

The warning message from nfsd terminating normally
can confuse system adminstrators or monitoring software.

Though it's not exactly fair to pin-point a commit where it
originated, the current form in the current place started
to appear in:

Fixes: e096bbc6 ("knfsd: remove special handling for SIGHUP")
Signed-off-by: kazuo ito <kzpn200@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>