Only a small subset of mmap(2)'s flags should be used in combination with
the flag MAP_GUARD. Rather than enumerating the flags that are not
allowed, enumerate the flags that are allowed. The list of allowed flags
is much shorter and less likely to change. (As an aside, one of the
previously enumerated flags, MAP_PREFAULT, was not even a legal flag for
mmap(2). However, because of an earlier check within kern_mmap(), this
misuse of MAP_PREFAULT was harmless.)

Reviewed by: kib
MFC after: 10 days