Unspammed nested include of <sys/malloc.h>. <sys/mbuf.h> hasn't
really used any features from <sys/malloc.h> since it was "blasted
from the past" in rev.1.15, and most .c files did not depend on
the namespace pollution (others have been fixed). The mbuf allocation
control values M_DONTWAIT and M_WAIT were just defined in terms of
the malloc control values M_NOWAIT and M_WAITOK. The mbuf values
are now defined directly.