Adds implicit updates of cached diagnostics via Exchange Config Data when
reading sysfs attributes interfacing them. Right now this only affects the
new B2B-Credit diagnostic attribute.

This uses the same mechanism previously also used for cached diagnostics
of Exchange Port Data.

Link: https://lore.kernel.org/r/60a94f55f2630b74b468fed5f39880208abb2679.1572018132.git.bblock@linux.ibm.comReviewed-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>