dm mpath: use more error codes

This patch allows path errors from the multipath ctr function to
propagate up to userspace as errno values from the ioctl() call.

This is in response to
  https://www.redhat.com/archives/dm-devel/2008-May/msg00000.html
and
  https://bugzilla.redhat.com/show_bug.cgi?id=444421

The patch only lets through the errors that it needs to in order to
get the path errors from parse_path().

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>