sunrpc/cache: use list_del_init for the list_head entries in cache_deferred_req

Using list_del_init is generally safer than list_del, and it will
allow us, in a subsequent patch, to see if an entry has already been
processed or not.

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