md/raid10: reset the 'first' at the end of loop

commit 6f287ca6046edd34ed83aafb7f9033c9c2e809e2 upstream.

We need to set "first = 0' at the end of rdev_for_each
loop, so we can get the array's min_offset_diff correctly
otherwise min_offset_diff just means the last rdev's
offset diff.

[only the first chunk, due to b506335e5d2b ("md/raid10: skip spare disk as
'first' disk") being already applied - gregkh]

Suggested-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>