md/raid10: extend r10bio devs to raid disks

md/raid10: extend r10bio devs to raid disks

BugLink: https://bugs.launchpad.net/bugs/1896578
Now it allocs r10bio->devs[conf->copies]. Discard bio needs to submit
to all member disks and it needs to use r10bio. So extend to
r10bio->devs[geo.raid_disks].

Reviewed-by: Coly Li <colyli@suse.de>
Tested-by: Adrian Huang <ahuang12@lenovo.com>
Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
(cherry picked from commit c2968285925adb97b9aa4ede94c1f1ab61ce0925)
Signed-off-by: Matthew Ruffell <matthew.ruffell@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>