mds: use inode_t::layout for dir layout policy

This cherry-pick is going in the reverse direction of normal. That's
because this direction makes for the minimal change -- this patchset
is required to fix the loss of directory layouts we were previously
seeing, but fixing it requires changing the encoding versions. So we
wrote it on top of Bobtail and let it update the struct_v's as they existed
then. Note that we here change a few encoding versions in ways which are
NOT COMPATIBLE with previous development code (but not any releases). In
particular, development code introduced and this removes the
file_layout_policy_t, and some of the CInode and EMetaBlob encoding
struct_v values were used in development code to mean one thing, but
mean something different due to the Bobtail patch.

Remove the default_file_layout struct, which was just a ceph_file_layout,
and store it in the inode_t.  Rip out all the annoying code that put this
on the heap.

To aid in this usage, add a clear_layout() function to inode_t.

Signed-off-by: Sage Weil <sage.weil@dreamhost.com>
Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 36ed407e0f939a9bca57c3ffc0ee5608d50ab7ed)
Conflicts:

	src/mds/CInode.cc
	src/mds/CInode.h
	src/mds/MDCache.cc
	src/mds/Server.cc
	src/mds/events/EMetaBlob.h
Cherry-pick-
Reviewed-by: Sage Weil <sage@inktank.com>