Require (reasonably) normal mappings for MADV_DOFORK

This came up as a result of the tracing fix pull request, and commit
e39bb9e02b68 ("tracing: Fix WARN_ON in tracing_buffers_mmap_close") in
particular.

The use of MADV_DOFORK confused the ring buffer mapping reference
counting just because it was unexpected, since the mapping was
originally done with VM_DONTCOPY.

The tracing code may well be the only case of this (and fixed it all by
just using the mmap open callback to unconfuse itself), but it's just
strange that we allow MADV_DOFORK on special mappings where the kernel
has set the "don't copy this" bit.

The code already disallowed it for VM_IO mappings (going back to the
original commit f822566165dd: "madvise MADV_DONTFORK/MADV_DOFORK"), so
just extend it to any of the VM_SPECIAL cases (which includes
VM_DONTEXPAND | VM_PFNMAP | VM_MIXEDMAP in addition to VM_IO).

We could also allow MADV_DOFORK only on mappings that had been marked
DONTFORK by the user. But that would require us to track that
(presumably with another VM_xyz bit), so let's just do this trivial and
straightforward modifications.

If anybody notices, Lorenzo will be boarding Flying Pig Airlines.

Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Link: https://lore.kernel.org/all/a8907468-d7e9-4727-af28-66d905093230@kernel.org/
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>