library: make struct name more descriptive, <STAT> api

library: make struct name more descriptive, <STAT> api

After normalizing the <SLABINFO> module, much of which
was based on this module, a structure which is crucial
to concurrent select/reap support is renamed the same.

Such a need for concurrent support was revealed during
slabtop testing. That program requests (select) global
data after calling for individual node data (reap) but
before those stacks have been accessed. Therefore, one
set of 'items'/'extents' could not possibly be shared.

So now we will know the former struct fetch_support as
ext_support. The former name was potentially confusing
since the struct was used by both 'select' and 'reap'.
However, only 'reap' ever called the 'fetch' function.

[ this <STAT> already accommodated concurrent usage. ]
[ in fact it contains 3 separate sets of items/exts. ]

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