sunrpc: don't wait for write before allowing reads from use-gss-proxy file

It doesn't make much sense to make reads from this procfile hang. As
far as I can tell, only gssproxy itself will open this file and it
never reads from it. Change it to just give the present setting of
sn->use_gss_proxy without waiting for anything.

Note that we do not want to call use_gss_proxy() in this codepath
since an inopportune read of this file could cause it to be disabled
prematurely.

Cc: stable@vger.kernel.org
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>