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>