[PATCH] Fix potential OOPs in blkdev_open()

blkdev_open() calls bc_acquire() to get a struct block_device.  Since
bc_acquire() may return NULL when system is out of memory an appropriate
check is required.

Signed-off-by: Pavel Emelianov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>