mds: parse ceph.*.layout vxattr key/value content

Use qi to parse a strictly formatted set of key/value pairs.  Be picky
about whitespace.  Any subset of recognized keys is allowed.  Parse the
same set of keys as the ceph.*.layout.* vxattrs.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 5551aa5b3b5c2e9e7006476b9cd8cc181d2c9a04)
mds: use inode_t::layout for dir layout policy

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>