18600332 ("mm: make PR_SET_THP_DISABLE immediately active") introduced
a regression in that userspace cannot always determine the set of vmas
where thp is disabled.

Userspace relies on the "nh" flag being emitted as part of /proc/pid/smaps
to determine if a vma has been disabled from being backed by hugepages.

Previous to this commit, prctl(PR_SET_THP_DISABLE, 1) would cause thp to
be disabled and emit "nh" as a flag for the corresponding vmas as part of
/proc/pid/smaps.  After the commit, thp is disabled by means of an mm flag
and "nh" is not emitted.

This causes smaps parsing libraries to assume a vma is enabled for thp and
ends up puzzling the user on why its memory is not backed by thp.

This also clears the "hg" flag to make the behavior of MADV_HUGEPAGE and
PR_SET_THP_DISABLE definitive.

Link: http://lkml.kernel.org/r/alpine.DEB.2.21.1809251449060.96762@chino.kir.corp.google.com
Fixes: 18600332

 ("mm: make PR_SET_THP_DISABLE immediately active")
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: <stable@vger.kernel.org>	[4.13+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>