nfsd4: fix cr_principal comparison check in same_creds

This fixes a wrong check for same cr_principal in same_creds

Introduced by 8fbba96e

 "nfsd4: stricter
cred comparison for setclientid/exchange_id".

Cc: stable@vger.kernel.org
Signed-off-by: Vivek Trivedi <vtrivedi018@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>