Define the cachefiles_cache struct that's going to carry the cache-level
parameters and state of a cache.

Define the beginning of the cachefiles_object struct that's going to carry
the state for a data storage object.  For the moment this is just a
debugging ID for logging purposes.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819623690.215744.2824739137193655547.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906924292.143852.15881439716653984905.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967131405.1823006.4480555941533935597.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021530610.640689.846094074334176928.stgit@warthog.procyon.org.uk/ # v4