Btrfs, scrub: uninitialized variable in scrub_extent_for_parity()

The only way that "ret" is set is when we call scrub_pages_for_parity()
so the skip to "if (ret) " test doesn't make sense and causes a static
checker warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>