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/readdir.c

@@ -44,7 +44,7 @@ int iterate_dir(struct file *file, struct dir_context *ctx)
 		fsnotify_access(file);
 		file_accessed(file);
 	}
-	mutex_unlock(&inode->i_mutex);
+	inode_unlock(inode);
 out:
 	return res;
 }