fscache: Add a cookie debug ID and use that in traces

Add a cookie debug ID and use that in traces and in procfiles rather than
displaying the (hashed) pointer to the cookie.  This is easier to correlate
and we don't lose anything when interpreting oops output since that shows
unhashed addresses and registers that aren't comparable to the hashed
values.

Changes:

ver #2:
 - Fix the fscache_op tracepoint to handle a NULL cookie pointer.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/158861210988.340223.11688464116498247790.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/159465769844.1376105.14119502774019865432.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/160588459097.3465195.1273313637721852165.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/162431193544.2908479.17556704572948300790.stgit@warthog.procyon.org.uk/