BUG/MINOR: mworker: deinit of thread poller was called when not initialized

BUG/MINOR: mworker: deinit of thread poller was called when not initialized

Commit 67e371e ("BUG/MEDIUM: mworker: FD leak of the eventpoll in wait
mode") introduced a regression. Upon a reload it tries to deinit the
poller per thread, but no poll loop was initialized after loading the
configuration.

This patch fixes the issue by moving this part of the code in
mworker_reload(), since this function will be called only when the
poller is fully initialized.

This patch must be backported in 2.5.

(cherry picked from commit efd954793e60131932c61883d0869c7912a5d12c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>