cachefiles: Explain checks in a comment

Add a comment to explain the checks that cachefiles is making of the
backing filesystem[1].
Suggested-by: Jeff Layton <jlayton@kernel.org>
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/568749bd7cc02908ecf6f3d6a611b6f9cf5c4afd.camel@kernel.org/ [1]
Link: https://lore.kernel.org/r/164251405621.3435901.771439791811515914.stgit@warthog.procyon.org.uk/ # v1

fs/cachefiles/cache.c

@@ -49,7 +49,13 @@ int cachefiles_add_cache(struct cachefiles_cache *cache)
 		goto error_unsupported;
 	}
 
-	/* check parameters */
+	/* Check features of the backing filesystem:
+	 * - Directories must support looking up and directory creation
+	 * - We use xattrs to store metadata
+	 * - We need to be able to query the amount of space available
+	 * - We want to be able to sync the filesystem when stopping the cache
+	 * - We use DIO to/from pages, so the blocksize mustn't be too big.
+	 */
 	ret = -EOPNOTSUPP;
 	if (d_is_negative(root) ||
 	    !d_backing_inode(root)->i_op->lookup ||