keys: Export lookup_user_key to external users

Export lookup_user_key() symbol in order to allow nvdimm passphrase
update to retrieve user injected keys.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index d5b25e5..ec4fd45 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -755,6 +755,7 @@ key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
 	put_cred(ctx.cred);
 	goto try_again;
 }
+EXPORT_SYMBOL(lookup_user_key);
 
 /*
  * Join the named keyring as the session keyring if possible else attempt to