KEYS: Fix some sparse warnings

Fix some sparse warnings in the keyrings code:

 (1) compat_keyctl_instantiate_key_iov() should be static.

 (2) There were a couple of places where a pointer was being compared against
     integer 0 rather than NULL.

 (3) keyctl_instantiate_key_common() should not take a __user-labelled iovec
     pointer as the caller must have copied the iovec to kernel space.

 (4) __key_link_begin() takes and __key_link_end() releases
     keyring_serialise_link_sem under some circumstances and so this should be
     declared.

     Note that adding __acquires() and __releases() for this doesn't help cure
     the warnings messages - something only commenting out both helps.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>

security/keys/keyctl.c

@@ -1106,7 +1106,7 @@ long keyctl_instantiate_key_iov(key_serial_t id,
 	struct iovec iovstack[UIO_FASTIOV], *iov = iovstack;
 	long ret;
 
-	if (_payload_iov == 0 || ioc == 0)
+	if (!_payload_iov || !ioc)
 		goto no_payload;
 
 	ret = rw_copy_check_uvector(WRITE, _payload_iov, ioc,