bdev: move ->bd_has_subit_bio to ->__bd_flags

In bdev_alloc() we have all flags initialized to false, so
assignment to ->bh_has_submit_bio n there is a no-op unless
we have partno != 0 and flag already set on entire device.

In device_add_disk() we have just allocated the block_device
in question and it had been a full-device one, so the flag
is guaranteed to be still clear when we get to assignment.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>