wrappers for ->i_mutex access

parallel to mutex_{lock,unlock,trylock,is_locked,lock_nested},
inode_foo(inode) being mutex_foo(&inode->i_mutex).

Please, use those for access to ->i_mutex; over the coming cycle
->i_mutex will become rwsem, with ->lookup() done with it held
only shared.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/cachefiles/interface.c

@@ -446,7 +446,7 @@ static int cachefiles_attr_changed(struct fscache_object *_object)
 		return 0;
 
 	cachefiles_begin_secure(cache, &saved_cred);
-	mutex_lock(&d_inode(object->backer)->i_mutex);
+	inode_lock(d_inode(object->backer));
 
 	/* if there's an extension to a partial page at the end of the backing
 	 * file, we need to discard the partial page so that we pick up new
@@ -465,7 +465,7 @@ static int cachefiles_attr_changed(struct fscache_object *_object)
 	ret = notify_change(object->backer, &newattrs, NULL);
 
 truncate_failed:
-	mutex_unlock(&d_inode(object->backer)->i_mutex);
+	inode_unlock(d_inode(object->backer));
 	cachefiles_end_secure(cache, saved_cred);
 
 	if (ret == -EIO) {