Prevent simultaneous dt(4) open.

Prevent simultaneous dt(4) open.

Syskaller has hit the assertion "dtlookup(unit) == NULL" by opening
dt(4) device in two parallel threads.  Convert kassert into if
condition.  Move check that device is not used after sleep points
in malloc.  The list dtdev_list is protected by kernel lock which
is released during sleep.

Reported-by: syzbot+6d66c21f796c817948f0@syzkaller.appspotmail.com
OK miod@