Minor source code cleanup of page flags in mm/page_alloc.c.
Move the definition of the groups of bits to page-flags.h.

The purpose of this clean up is that the next patch will
conditionally add a page flag to the groups.  Doing that
in a header file is cleaner than adding #ifdefs to the
C code.
Signed-off-by: NRuss Anderson <rja@sgi.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>