NFS: Add a tracepoint to show the results of nfs_set_cache_invalid()

[ Upstream commit 93c2e5e0 ]

This provides some insight into the client's invalidation behavior to show
both when the client uses the helper, and the results of calling the
helper which can vary depending on how the helper is called.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Stable-dep-of: 867da60d ("nfs: avoid i_lock contention in nfs_clear_invalid_mapping")
Signed-off-by: Sasha Levin <sashal@kernel.org>