On various systems __unused is a define to __attribute__((__unused__))
and yields compile errors as a result. Use __spare instead to describe
that these bits are "unused".

Sponsored by:  The FreeBSD Foundation
Signed-off-by: Bjoern A. Zeeb <bz@FreeBSD.ORG>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>