mm/vmscan: restore zone_reclaim_mode ABI

I went to go add a new RECLAIM_* mode for the zone_reclaim_mode sysctl.
Like a good kernel developer, I also went to go update the
documentation.  I noticed that the bits in the documentation didn't
match the bits in the #defines.

The VM never explicitly checks the RECLAIM_ZONE bit.  The bit is,
however implicitly checked when checking 'node_reclaim_mode==0'.  The
RECLAIM_ZONE #define was removed in a cleanup.  That, by itself is fine.

But, when the bit was removed (bit 0) the _other_ bit locations also got
changed.  That's not OK because the bit values are documented to mean
one specific thing.  Users surely do not expect the meaning to change
from kernel to kernel.

The end result is that if someone had a script that did:

	sysctl vm.zone_reclaim_mode=1

it would have gone from enabling node reclaim for clean unmapped pages
to writing out pages during node reclaim after the commit in question.
That's not great.

Put the bits back the way they were and add a com...