mds: handle ceph.*.layout.* setxattr

Allow individual fields of file or dir layouts to be set via setxattr.

Signed-off-by: Sage Weil <sage@inktank.com>
mds: allow dir layout/policy to be removed via removexattr on ceph.dir.layout

This lets a user remove a policy that was previously set on a dir.

Signed-off-by: Sage Weil <sage@inktank.com>
qa: add layout_vxattrs.sh test script

Test virtual xattrs for file and directory layouts.

TODO: create a data pool, add it to the fs, and make sure we can use it.

Signed-off-by: Sage Weil <sage@inktank.com>