Fix: run_as_mkdirat returns value of errno

The run_as_mkdirat() function returns the _errno value returned
by the run-as worker instead of returning `ret`. This causes
run_as_mkdirat() to return an error when errno was already set
to a non-zero value by a previous libc/system call.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>