nfsd4: separate delegreturn case from preprocess_stateid_op

Delegreturn is enough a special case for preprocess_stateid_op to
warrant just open-coding it in delegreturn.

There should be no change in behavior here; we're just reshuffling code.

Thanks to Yang Hongyang for catching a critical typo.

Reviewed-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>