monitor: Fix return type of monitor_fdset_dup_fd_find

monitor_fdset_dup_fd_find_remove() and monitor_fdset_dup_fd_find()
return mon_fdset->id which is int64_t. Downcasting from int64_t to int
leads to a bug with removing fd from fdset with id >= 2^32.
So, fix return types for these function.

Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190523094433.30297-1-yury-kotov@yandex-team.ru>
Signed-off-by: Markus Armbruster <armbru@redhat.com>