cachefiles: Change %p in format strings to something else

Change plain %p in format strings in cachefiles code to something more
useful, since %p is now hashed before printing and thus no longer matches
the contents of an oops register dump.

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/160588476042.3465195.6837847445880367183.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/162431200692.2908479.9253374494073633778.stgit@warthog.procyon.org.uk/