mm/z3fold.c: limit first_num to the actual range of possible buddy indexes

mm/z3fold.c: limit first_num to the actual range of possible buddy indexes

At present, Tying the first_num size to NCHUNKS_ORDER is confusing.  the
number of chunks is completely unrelated to the number of buddies.

The patch limits the first_num to actual range of possible buddy indexes.
and that is more reasonable and obvious without functional change.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: zhong jiang <[email protected]>
Suggested-by: Dan Streetman <[email protected]>
Acked-by: Dan Streetman <[email protected]>
Acked-by: Vitaly Wool <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>