ceph: let the set_layout ioctl set single traits

Previously we were validating the passed-in stripe unit, object size,
and stripe count against each other (and not testing most other stuff).
Instead, make sure that the composed previous layout and new values are valid,
and only send the new values to the MDS. This lets users change the
pool without setting the whole layout, for instance.

Signed-off-by: Greg Farnum <gregory.farnum@dreamhost.com>