specfs: Call bdev_open without the vnode lock.

There is no need for it to serialize opens, because they are already
serialized by sd_opencnt which for block devices is always either 0
or 1.

There's not obviously any other reason why the vnode lock should be
held across bdev_open, other than that it might be nice to avoid
dropping it if not necessary.  For character devices we always have
to drop the vnode lock because open might hang indefinitely, when
opening a tty, which is not allowed while holding the vnode lock.

(riastradh)