MINOR: fd/threads: make _GET_NEXT()/_GET_PREV() use the volatile attribute

MINOR: fd/threads: make _GET_NEXT()/_GET_PREV() use the volatile attribute

These macros are either used between atomic ops which cause the volatile
to be implicit, or with an explicit volatile cast. However not having it
in the macro causes some traps in the code because certain loop paths
cannot safely be used without risking infinite loops if one isn't careful
enough.

Let's place the volatile attribute inside the macros and remove them from
the explicit places to avoid this. It was verified that the output executable
remains exactly the same byte-wise.

(cherry picked from commit 337fb719ee93776cb399a333e04e2438427f0111)
Signed-off-by: Willy Tarreau <w@1wt.eu>