libmultipath/discovery.c: Fix a condition that will lead to a table reload during every check.

If path_offline says PATH_UP and the checker says PATH_DOWN.  In
this case, pathinfo gets called twice, and the first call, from
update_prio sets it to -1 because the checker says it's down.  On the
second pathinfo call, from update_path_groups, it will call the
prioritizer based on the fact that the prio is -1. This leads to a flip
flop of the priority value and a reload on every check.

Signed-off-by: Sean Stewart <sean.stewart@netapp.com>
libmultipath/checkers/rdac.c: Use RTPG data in RDAC checker

Make the RDAC checker utilize the RTPG data given in the 0xC9 inquiry
page to make intelligent decisions about path status, and print more
descriptive path down messages.

Signed-off-by: Sean Stewart <sean.stewart@netapp.com>