commit [1] causes missing bitmap updates when there isn't any WriteMostly
devices.

Detailed steps to reproduce by Norbert (which somehow didn't make to lore):

   # setup md10 (raid1) with two drives (1 GByte sparse files)
   dd if=/dev/zero of=disk1 bs=1024k seek=1024 count=0
   dd if=/dev/zero of=disk2 bs=1024k seek=1024 count=0

   losetup /dev/loop11 disk1
   losetup /dev/loop12 disk2

   mdadm --create /dev/md10 --level=1 --raid-devices=2 /dev/loop11 /dev/loop12

   # add bitmap (aka write-intent log)
   mdadm /dev/md10 --grow --bitmap=internal

   echo check > /sys/block/md10/md/sync_action

   root:# cat /sys/block/md10/md/mismatch_cnt
   0
   root:#

   # remove member drive disk2 (loop12)
   mdadm /dev/md10 -f loop12 ; mdadm /dev/md10 -r loop12

   # modify degraded md device
   dd if=/dev/urandom of=/dev/md10 bs=512 count=1

   # no blocks recorded as out of sync on the remaining member disk1/loop11
   root:# mdadm -X /dev/loop11 | grep Bitmap
             Bitmap : 16 bits (chunks), 0 dirty (0.0%)
   root:#

   # re-add disk2, nothing synced because of empty bitmap
   mdadm /dev/md10 --re-add /dev/loop12

   # check integrity again
   echo check > /sys/block/md10/md/sync_action

   # disk1 and disk2 are no longer in sync, reads return differend data
   root:# cat /sys/block/md10/md/mismatch_cnt
   128
   root:#

   # clean up
   mdadm -S /dev/md10
   losetup -d /dev/loop11
   losetup -d /dev/loop12
   rm disk1 disk2

Fix this by moving the WriteMostly check to the if condition for
alloc_behind_master_bio().

[1] commit fd3b6975 ("md/raid1: only allocate write behind bio for WriteMostly device")
Fixes: fd3b6975 ("md/raid1: only allocate write behind bio for WriteMostly device")
Cc: stable@vger.kernel.org # v5.12+
Cc: Guoqing Jiang <guoqing.jiang@linux.dev>
Cc: Jens Axboe <axboe@kernel.dk>
Reported-by: Norbert Warmuth <nwarmuth@t-online.de>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Song Liu <song@kernel.org>