md raid1: allow writebehind to work on any leg device set WriteMostly

As the WriteMostly flag can be set on any component device of a RAID1
array, remove the constraint that it only works if set on the first one.

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Tested-by: Xiao Ni <xni@redhat.com>
Link: https://lore.kernel.org/r/2a9592bf3340f34bf588eec984b23ee219f3985e.1692013451.git.heinzm@redhat.com
Signed-off-by: Song Liu <song@kernel.org>