Temporarily check for and correct a race in getnewbuf() that exists due

Temporarily check for and correct a race in getnewbuf() that exists due
to the fact that lockmgr locks use tokens for their interlock.  The use
of a token can cause the atomicy of the big giant lock to be temporarily
lost and wind up breaking the assumed atomicy of higher level operations that
believed themselves to be safe making lockmgr calls with the LK_NOWAIT flag.

The general problem will soon be fixed by changing the lockmgr interlock
from a token to one of Jeffrey Hsu's spin locks.  Fortunately there are
only a few places left in DragonFly where LK_INTERLOCK is used.