nfsd: add trace events

For now just a few simple events to trace the layout stateid lifetime, but
these already were enough to find several bugs in the Linux client layout
stateid handling.

Signed-off-by: Christoph Hellwig <hch@lst.de>

diff --cc fs/nfsd/Makefile
Simple merge

diff --cc fs/nfsd/nfs4layouts.c
Simple merge

diff --cc fs/nfsd/nfs4proc.c
Simple merge

diff --cc fs/nfsd/trace.c
Simple merge

diff --cc fs/nfsd/trace.h
Simple merge