nfsd: protect concurrent access to nfsd stats counters

nfsd: protect concurrent access to nfsd stats counters

[ Upstream commit e567b98ce9a4b35b63c364d24828a9e5cd7a8179 ]

nfsd stats counters can be updated by concurrent nfsd threads without any
protection.

Convert some nfsd_stats and nfsd_net struct members to use percpu counters.

The longest_chain* members of struct nfsd_net remain unprotected.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>