mm/swapfile.c: classify SWAP_MAP_XXX to make it more readable

mm/swapfile.c: classify SWAP_MAP_XXX to make it more readable

swap_info_struct->swap_map[] encodes some flag and count. And to
do some condition check, it also introduces some special values.

Currently those macros are defined with some magic order, which makes
audience hard to understand the exact meaning.

This patch split those macros into three categories:

    flag
    special value for first swap_map
    special value for continued swap_map

May this help audiences a little.

[akpm@linux-foundation.org: tweak capitalization in comments]
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200501015259.32237-1-richard.weiyang@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>