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/kernfs/dir.c

@@ -1511,9 +1511,9 @@ static loff_t kernfs_dir_fop_llseek(struct file *file, loff_t offset,
 	struct inode *inode = file_inode(file);
 	loff_t ret;
 
-	mutex_lock(&inode->i_mutex);
+	inode_lock(inode);
 	ret = generic_file_llseek(file, offset, whence);
-	mutex_unlock(&inode->i_mutex);
+	inode_unlock(inode);
 
 	return ret;
 }