cachefiles: Implement metadata/coherency data storage in xattrs

Use an xattr on each backing file in the cache to store some metadata, such
as the content type and the coherency data.

Five content types are defined:

 (0) No content stored.

 (1) The file contains a single monolithic blob and must be all or nothing.
     This would be used for something like an AFS directory or a symlink.

 (2) The file is populated with content completely up to a point with
     nothing beyond that.

 (3) The file has a map attached and is sparsely populated.  This would be
     stored in one or more additional xattrs.

 (4) The file is dirty, being in the process of local modification and the
     contents are not necessarily represented correctly by the metadata.
     The file should be deleted if this is seen on binding.

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/163819641320.215744.16346770087799536862.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906942248.143852.5423738045012094252.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967151734.1823006.9301249989443622576.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021550471.640689.553853918307994335.stgit@warthog.procyon.org.uk/ # v4