btrfs-progs: mkfs/rootdir: Fix memory leak in traverse_directory()

btrfs-progs: mkfs/rootdir: Fix memory leak in traverse_directory()

The bug is exposed by mkfs test case 009, with D=asan.

We are leaking memory of parent_dir_entry->path() which ,except the
rootdir, is allocated by strdup().

Before fixing it, unifiy the allocation of parent_dir_entry() to heap
allocation.

Then fix it by adding "free(parent_dir_entry->path);" in
traverse_directory() and error handler.

Issue: #92
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>