Staging: lustre: Convert macro class_export_rpc_dec into static inline function

This patch converts the macro class_export_rpc_dec into static inline
function. This is possible because the types of arguments at all the call
sites are same. So, the type of parameter is compatible with the types of
the arguments at all of the call sites.

Signed-off-by: Somya Anand <somyaanand214@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>