KEYS: get_instantiation_keyring() should inc the keyring refcount in all cases

The destination keyring specified to request_key() and co. is made available to
the process that instantiates the key (the slave process started by
/sbin/request-key typically).  This is passed in the request_key_auth struct as
the dest_keyring member.

keyctl_instantiate_key and keyctl_negate_key() call get_instantiation_keyring()
to get the keyring to attach the newly constructed key to at the end of
instantiation.  This may be given a specific keyring into which a link will be
made later, or it may be asked to find the keyring passed to request_key().  In
the former case, it returns a keyring with the refcount incremented by
lookup_user_key(); in the latter case, it returns the keyring from the
request_key_auth struct - and does _not_ increment the refcount.

The latter case will eventually result in an oops when the keyring prematurely
runs out of references and gets destroyed.  The effect may take some time to
show up as t...