Fix return value of userns_exec_1

Instead of always returning -1 and call SYSERROR when the child returns
non-zero. Have userns_exec_1 always return the return value from the
function it's calling and let the caller do the error handling (as is
already done by its only caller).

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>