On LPAR the read message buffer command should be executed on the path
it was received on otherwise there is a chance that the CUIR assignment
might be faulty and the wrong channel path is set online/offline.

Fix by setting the path mask accordingly.
On z/VM we might not be able to do I/O on this path but there it does
not matter on which path the read message buffer command is executed.
Therefor implement a retry with an open path mask.
Signed-off-by: Stefan Haberland <sth@linux.vnet.ibm.com>