fscache: Change %p in format strings to something else

Change plain %p in format strings in fscache 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/160588474843.3465195.5446072310069374803.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/162431199509.2908479.2950631488219944294.stgit@warthog.procyon.org.uk/