Fix: provide errno as argument to urcu_die()

commit 1a990de3add "Fix: rculfhash worker needs to unblock to SIGRCU"
provides "ret" (-1) as argument to urcu_die(), but should rather provide
errno.

Reported by Coverity:

** CID 1405700:  Error handling issues  (NEGATIVE_RETURNS) /src/rculfhash.c: 2171 in cds_lfht_worker_init()

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>