security/keys: Remove inconsistent __user annotation

The declaration of keyring_read does not match the definition
(security/keys/keyring.c). In this case the definition is correct
because it matches what defined in "struct key_type::read"
(linux/key-type.h).

Fix the declaration removing the inconsistent __user annotation.

Cc: David Howells <dhowells@redhat.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Paul Moore <paul@paul-moore.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>