include/linux/gfp.h: convert BUG_ON() into VM_BUG_ON()

VM_BUG_ON() if effectively a BUG_ON() undef #ifdef CONFIG_DEBUG_VM.  That
is exactly what we have here now, and two different folks have suggested
doing it this way.

Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>