LU-10171 headers: define pct(a,b) once

LU-10171 headers: define pct(a,b) once

pct is defined 6 times in different places.  Define it in one.
Also change it to a static inline to do a better job of
enforcing types.

Signed-off-by: Ben Evans <bevans@cray.com>
Change-Id: If61132a1096c351a9bcb7debb868351206267535
Reviewed-on: https://review.whamcloud.com/29852
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>