drivers/mmc/card/block.c: fix refcount leak in mmc_block_open()

drivers/mmc/card/block.c: fix refcount leak in mmc_block_open()

mmc_block_open() increments md->usage although it returns with -EROFS when
default mounting a MMC/SD card with write protect switch on.  This
reference counting bug prevents /dev/mmcblkX from being released on card
removal, and situation worsen with reinsertion until the minor number
range runs out.

Reported-by: <[email protected]>
Acked-by: Pierre Ossman <[email protected]>
Cc: <[email protected]>		[2.6.25.x, 2.6.26.x]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>