md/raid10: skip spare disk as 'first' disk

Commit 6f287ca6(md/raid10: reset the 'first' at the end of loop) ignores
a case in reshape, the first rdev could be a spare disk, which shouldn't
be accounted as the first disk since it doesn't include the offset info.

Fix: 6f287ca6(md/raid10: reset the 'first' at the end of loop)
Cc: Guoqing Jiang <gqjiang@suse.com>
Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>