nfsd4: fix mixed 4.0/4.1 handling, 4.1 reboot

Instead of failing to find client entries which don't match the
minorversion, we should be finding them, then either erroring out or
expiring them as appropriate.

This also fixes a problem which would cause the 4.1 server to fail to
recognize clients after a second reboot.

Reported-by: Casey Bodley <cbodley@citi.umich.edu>
Reviewed-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>