KEYS: Use a typedef for restrict_link function pointers

This pointer type needs to be returned from a lookup function, and
without a typedef the syntax gets cumbersome.

Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index 3d95f7d..1b29ac7 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -492,9 +492,7 @@ static long keyring_read(const struct key *keyring,
 struct key *keyring_alloc(const char *description, kuid_t uid, kgid_t gid,
 			  const struct cred *cred, key_perm_t perm,
 			  unsigned long flags,
-			  int (*restrict_link)(struct key *,
-					       const struct key_type *,
-					       const union key_payload *),
+			  key_restrict_link_func_t restrict_link,
 			  struct key *dest)
 {
 	struct key *keyring;