sysctl: Correct error return from get_subdir

When insert_header fails ensure we return the proper error value
from get_subdir.  In practice nothing cares, but there is no
need to be sloppy.

Reported-by: Lucian Adrian Grijincu <lucian.grijincu@gmail.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>