nfsd: add tracepoint to nfsd_readdir

nfsd: add tracepoint to nfsd_readdir

Observe the start of NFS READDIR operations.

The NFS READDIR's count argument can be interesting when tuning a
client's readdir behavior.

However, the count argument is not passed to nfsd_readdir(). To
properly capture the count argument, this tracepoint must appear in
each proc function before the nfsd_readdir() call.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>