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

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

[ Upstream commit 6f287ca6046edd34ed83aafb7f9033c9c2e809e2 ]

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.

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>