NFS: directory trace messages

Reuse NFSDBG_DIRCACHE and NFSDBG_LOOKUPCACHE to provide additional
diagnostic messages that trace the operation of the NFS client's
directory behavior.  A few new messages are now generated when NFSDBG_VFS
is active, as well, to trace normal VFS activity.  This compromise
provides better trace debugging for those who use pre-built kernels,
without adding a lot of extra noise to the standard debug settings.

Test-plan:
Enable NFS trace debugging with flags 1, 2, or 4.  You should be able to
see different types of trace messages with each flag setting.

Signed-off-by: Chuck Lever <cel@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>