Fix reshape for decreasing data offset

Fix reshape for decreasing data offset

...when not changing the number of disks.

This patch needs context to explain. These are the relevant parts of
the original code (condensed and annotated):

if (dir > 0) {
    /* Increase data offset (reshape backwards) */
    if (data_offset < sd->data_offset + min) {
        pr_err("--data-offset too small on %s\n",
               dn);
        goto release;
    }
} else {
    /* Decrease data offset (reshape forwards) */
    if (data_offset < sd->data_offset - min) {
        pr_err("--data-offset too small on %s\n",
               dn);
        goto release;
    }
}

When this code is reached, mdadm has already decided on a reshape
direction. When increasing the data offset, the reshape runs backwards
(dir==1); when decreasing the data offset, the reshape runs forwards
(dir==-1).

The conditional within the backwards reshape is correct: the requested
offset must be larger than the old offset plus a minimum delta; thus the
reshape has room to work.

For the forwards reshape, the requested offset needs to be smaller than
the old offset minus a minimum delta; to do this correctly, the
comparison must be reversed.

Also update the error message.

Note: I have tested this change on a RAID 5 on Linux 4.18.0 and verified
that there were no errors from the kernel and that the device data
remained intact. I do not know if there are considerations for different
RAID levels.

Signed-off-by: Corey Hickey <bugfood-c@fatooh.org>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>