include/linux/mmdebug.h: make VM_WARN* non-rvals

At present the construct

	if (VM_WARN(...))

will compile OK with CONFIG_DEBUG_VM=y and will fail with
CONFIG_DEBUG_VM=n.  The reason is that VM_{WARN,BUG}* have always been
special wrt.  {WARN/BUG}* and never generate any code when DEBUG_VM is
disabled.  So we cannot really use it in conditionals.

We considered changing things so that this construct works in both cases
but that might cause unwanted code generation with CONFIG_DEBUG_VM=n.
It is safer and simpler to make the build fail in both cases.

[akpm@linux-foundation.org: changelog]
Signed-off-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>