block: fix booting from partitioned md array

Hi Tejun,

 it looks like your commit:

   block: don't depend on consecutive minor space
   f331c029

 broke a particular case for booting from partitioned md/raid devices.
 That is the second time this has been broken recently.  The previous
 time was fixed by

   block: do_mounts - accept root=<non-existant partition>
   30f2f0eb

 Because the data isn't available when an md device is first created
 (we add disks and set it up after creation), the initial partition
 scan finds nothing.  It is not until the device is opened that
 another partition scan happens and finds something.

 So at the point where the kernel parameter "root=/dev/md_d0p1" is
 being parsed, md_d0 exists, but md_d0p1 does not.
 However if we let blk_lookup_devt return the correct device number
 even though the device doesn't exist, then the attempt to mount it
 will successfully find the partition.

 I have tried in the past to find a way to get the partition table to
 be read as soon as the array is assembled but that proved impossible
 (at the time).  I don't remember the details, and could possibly
 revisit it.  However it would be really nice if blk_lookup_devt
 could be adjusted to again accept non existant partitions.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>