library: <diskstats> api, regularize macro conventions

library: <diskstats> api, regularize macro conventions

This cosmetic change just prepares for the later patch
introducing validation of Item_tables and enumerators.

[ and, we'll now have better 'set' function names of ]
[ 'set_diskstats_ENUM' instead of the more redundant ]
[ current 'set_diskstats_DISKSTATS_ENUM' convention. ]

[ now our only exception is the <slabinfo> api where ]
[ a full enumerator identifier with 'SLAB' & 'SLABS' ]
[ prefixes are used, and 'SLABINFO_noop/extra' guys. ]

Signed-off-by: Jim Warner <james.warner@comcast.net>