btrfs: fix an error handling path in btrfs_defrag_leaves()

All error handling paths end to 'out', except this memory allocation
failure.

This is spurious. So branch to the error handling path also in this case.
It will add a call to:

	memset(&root->defrag_progress, 0,
	       sizeof(root->defrag_progress));

Fixes: 6702ed49 ("Btrfs: Add run time btree defrag, and an ioctl to force btree defrag")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>