ext4: teach ext4_ext_find_extent() to free path on error

Right now, there are a places where it is all to easy to leak memory
on an error path, via a usage like this:

	struct ext4_ext_path *path = NULL

	while (...) {
		...
		path = ext4_ext_find_extent(inode, block, path, 0);
		if (IS_ERR(path)) {
			/* oops, if path was non-NULL before the call to
			   ext4_ext_find_extent, we've leaked it!  :-(  */
			...
			return PTR_ERR(path);
		}
		...
	}

Unfortunately, there some code paths where we are doing the following
instead:

	path = ext4_ext_find_extent(inode, block, orig_path, 0);

and where it's important that we _not_ free orig_path in the case
where ext4_ext_find_extent() returns an error.

So change the function signature of ext4_ext_find_extent() so that it
takes a struct ext4_ext_path ** for its third argument, and by
default, on an error, it will free the struct ext4_ext_path, and then
zero out the struct ext4_ext_path * pointer.  In order to avoid
causing problems, we add a flag EXT4_EX_NOFREE_ON_ERR which causes
ext4_ext_find_extent() to use the original behavior of forcing the
caller to deal with freeing the original path pointer on the error
case.

The goal is to get rid of EXT4_EX_NOFREE_ON_ERR entirely, but this
allows for a gentle transition and makes the patches easier to verify.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/move_extent.c

@@ -37,7 +37,7 @@ get_ext_path(struct inode *inode, ext4_lblk_t lblock,
 	int ret = 0;
 	struct ext4_ext_path *path;
 
-	path = ext4_ext_find_extent(inode, lblock, *orig_path, EXT4_EX_NOCACHE);
+	path = ext4_ext_find_extent(inode, lblock, orig_path, EXT4_EX_NOCACHE);
 	if (IS_ERR(path))
 		ret = PTR_ERR(path);
 	else if (path[ext_depth(inode)].p_ext == NULL)