md/raid6: use valid sector values to determine if an I/O should wait on the reshape

During a reshape or a RAID6 array such as expanding by adding an additional
disk, I/Os to the region of the array which have not yet been reshaped can
stall indefinitely. This is from errors in the stripe_ahead_of_reshape
function causing md to think the I/O is to a region in the actively
undergoing the reshape.

stripe_ahead_of_reshape fails to account for the q disk having a sector
value of 0. By not excluding the q disk from the for loop, raid6 will always
generate a min_sector value of 0, causing a return value which stalls.

The function's max_sector calculation also uses min() when it should use
max(), causing the max_sector value to always be 0. During a backwards
rebuild this can cause the opposite problem where it allows I/O to advance
when it should wait.

Fixing these errors will allow safe I/O to advance in a timely manner and
delay only I/O which is unsafe due to stripes in the middle of undergoing
the reshape.

Fixes: 486f6055

 ("md/raid5: Check all disks in a stripe_head for reshape progress")
Cc: stable@vger.kernel.org # v6.0+
Signed-off-by: David Jeffery <djeffery@redhat.com>
Tested-by: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231128181233.6187-1-djeffery@redhat.com