knfsd: cache unregistration needn't return error

There's really nothing much the caller can do if cache unregistration
fails.  And indeed, all any caller does in this case is print an error
and continue.  So just return void and move the printk's inside
cache_unregister.

Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>