NFS: Improvements for fs_context-related tracepoints

Add some missing observability to the fs_context tracepoints
added by commit 33ce83ef

 ("NFS: Replace fs_context-related
dprintk() call sites with tracepoints").

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>