mm: fix warning in __set_page_dirty_nobuffers

New tmpfs use of !PageUptodate pages for fallocate() is triggering the
WARNING: at mm/page-writeback.c:1990 when __set_page_dirty_nobuffers()
is called from migrate_page_copy() for compaction.

It is anomalous that migration should use __set_page_dirty_nobuffers()
on an address_space that does not participate in dirty and writeback
accounting; and this has also been observed to insert surprising dirty
tags into a tmpfs radix_tree, despite tmpfs not using tags at all.

We should probably give migrate_page_copy() a better way to preserve the
tag and migrate accounting info, when mapping_cap_account_dirty().  But
that needs some more work: so in the interim, avoid the warning by using
a simple SetPageDirty on PageSwapBacked pages.

Reported-and-tested-by: Dave Jones <davej@redhat.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>