multipath: make path devices readonly again.

multipath: make path devices readonly again.

Path device fds were changed to be opened read/write when the
mpathpersist code was added. However, I have talked with Vijay, and
this doesn't appear to be necessary for mpathpersist to work correctly.
If the path fds are opened read/write, when the are closed a change
uevent is triggered, which was causing problems during shutdown with
LVM on top of multipath devices. This patch reverts them to being read-only
again.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>