btrfs: DEFINE_FREE for struct btrfs_path

Add a DEFINE_FREE for struct btrfs_path. This defines a function that
can be called using the __free attribute. Define a macro
BTRFS_PATH_AUTO_FREE to make the declaration of an auto freeing path
very clear.

The intended use is to define the auto free of path in cases where the
path is allocated somewhere at the beginning and freed either on all
error paths or at the end of the function.

  int func() {
	  BTRFS_PATH_AUTO_FREE(path);

	  if (...)
		  return -ERROR;

	  path = alloc_path();

	  ...

	  if (...)
		  return -ERROR;

	  ...
	  return 0;
  }

Signed-off-by: Leo Martins <loemra.dev@gmail.com>
[ update changelog ]
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>