struct btrfs_ioctl_dev_replace_args memory is leaked if replace is
requested on a read-only filesystem.  Fix it.
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>