NFSD: Streamline the rare "found" case

[ Upstream commit add1511c ]

Move a rarely called function call site out of the hot path.

This is an exceptionally small improvement because the compiler
inlines most of the functions that nfsd_cache_lookup() calls.

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