block: revert block_dev read-only check

This reverts commit 75f1dc0d ("block: check bdev_read_only() from
blkdev_get()").  That commit added stricter checking to make sure
devices that were being used read-only were actually opened in that
mode.

It turns out that the change breaks a bunch of kernel code that opens
block devices.  Affected systems include dm, md, and the loop device.
Because strict checking for read-only opens of block devices was not
done before this, the code that opens the devices was opening them
read-write even if they were being used read-only.  Auditing all that
code will take time, and new userspace packages for dm, mdadm, etc.
will also be required.

Signed-off-by: Chuck Ebbert <cebbert@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>