BUG/MEDIUM: lists: Lock the element while we check if it is in a list.

BUG/MEDIUM: lists: Lock the element while we check if it is in a list.

In MT_LIST_TRY_ADDQ() and MT_LIST_TRY_ADD() we can't just check if the
element is already in a list, because there's a small race condition, it
could be added  between the time we checked, and the time we actually set
its next and prev, so we have to lock it first.

This is required to address issue #958.

This should be backported to 2.3, 2.2 and 2.1.

(cherry picked from commit 1f05324cbe92a7dde71f44dc740eb8240539746f)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit c4f3013d9452707d4efbda455360c8d2af022411)
[wt: fix has been in 2.3 for 3 versions now]
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 4aebb4e88ed360003efacc7f75a48c1d39bda6bb)
[wt: updated the two macros to match the equivalent upstream patch
 because during the reorg they were cleaned up by trimming the unused
 do{}while(0), making the patch impossible to apply that way]
Signed-off-by: Willy Tarreau <w@1wt.eu>