cachefiles: Add cache error reporting macro

Add a macro to report a cache I/O error and to tell fscache that the cache
is in trouble.

Also add a pointer to the fscache cache cookie from the cachefiles_cache
struct as we need that to pass to fscache_io_error().

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/163819626562.215744.1503690975344731661.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906927235.143852.13694625647880837563.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967137158.1823006.2065038830569321335.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021536053.640689.5306822604644352548.stgit@warthog.procyon.org.uk/ # v4