Improve smb(4) devfs interactions.

Use make_dev_s(9) to create device, since the device ioctl interface
needs to access si_drv1 to get softc pointer.

Remove the common but not functional attempt to prevent parallel
accesses by file descriptors by blocking more than one open. Either
threads in one process, or forked siblings, or file descriptors passed
over unix domain sockets all allow to execute parallel requests once
one fd is opened. Since ioctl handler uses smbus_request_bus() to
take the bus ownership, the correct mechanism establishes exclusive
access already.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks