NFSD: Skip extra computation for RC_NOCACHE case

[ Upstream commit 0f29ce32 ]

Force the compiler to skip unneeded initialization for cases that
don't need those values. For example, NFSv4 COMPOUND operations are
RC_NOCACHE.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>