KEYS: remove unnecessary get/put of explicit dest_keyring

In request_key_and_link(), in the case where the dest_keyring was
explicitly specified, there is no need to get another reference to
dest_keyring before calling key_link(), then drop it afterwards.  This
is because by definition, we already have a reference to dest_keyring.

This change is useful because we'll be making
construct_get_dest_keyring() able to return an error code, and we don't
want to have to handle that error here for no reason.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>