Pointer 'newargs' is used after the memory that it points to has already
been freed.

Picked up by Coverity - CID 1201425.

Fixes: 0723a047 ("btrfs: allow mounting btrfs subvolumes with
different ro/rw options")
Signed-off-by: NChristoph Jaeger <christophjaeger@linux.com>
Signed-off-by: NChris Mason <clm@fb.com>