Staging: lustre: include: Convert macro class_export_lock_get into static inline function

Static inline functions are preferred over macros. This change is safe
because the types of arguments at all the call sites are same.

Signed-off-by: Ksenija Stanojevic <ksenija.stanojevic@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>