Restored used include of <sys/malloc.h>. malloc() is not used
here, but kmem_malloc() is used and it takes the same "flags" as
malloc().

Use the mbuf allocation "flags" M_WAIT and M_DONTWAIT consistently.
There is really only one boolean flag, M_DONTWAIT, but the "flags"
were always treated as enum-like values, except in some places here
where the values are tacitly converted to boolean flags. Treat
them as enum-like values everywhere, except where we tacitly assume
that there are only two values in order to convert them to the
corresponding two kmem_malloc() "flags".