NFSD: Add tracepoints to NFSD's duplicate reply cache

[ Upstream commit 0b175b18648ebedfe255b11a7792f1d76848a8f7 ]

Try to capture DRC failures.

Two additional clean-ups:
- Introduce Doxygen-style comments for the main entry points
- Remove a dprintk that fires for an allocation failure. This was
  the only dprintk in the REPCACHE class.

Reported-by: kbuild test robot <lkp@intel.com>
[ cel: force typecast for display of checksum values ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Stable-dep-of: 3bc8edc98bd4 ("nfsd: under NFSv4.1, fix double svc_xprt_put on rpc_create failure")
Signed-off-by: Sasha Levin <sashal@kernel.org>