mmc: block: Allow more than 8 partitions per card

It is quite common for Android devices to utilize more
then 8 partitions on internal eMMC storage.

The vanilla kernel can support this via
CONFIG_MMC_BLOCK_MINORS, however that solution caps the
system to 256 minors total, which limits the number of
mmc cards the system can support.

This patch, which has been carried for quite awhile in
the AOSP common tree, provides an alternative solution
that doesn't seem to limit the total card count. So I
wanted to submit it for consideration upstream.

This patch sets the GENHD_FL_EXT_DEVT flag, which will
allocate minor number in major 259 for partitions past
disk->minors.

It also removes the use of disk_devt to determine devidx
from md->disk. md->disk->first_minor is always initialized
from devidx and can always be used to recover it.

Cc: Ulf Hansson <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Ben Hutchings <[email protected]>
Cc: Chuanxiao Dong <[email protected]>
Cc: Shawn Lin <[email protected]>
Cc: Austin S Hemmelgarn <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Android Kernel Team <[email protected]>
Cc: [email protected]
Signed-off-by: Colin Cross <[email protected]>
[jstultz: Added context to commit message]
Signed-off-by: John Stultz <[email protected]>
Signed-off-by: Ulf Hansson <[email protected]>

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