client: fix frag selection code
client: fix frag selection code

Calling fragtree_t::contains() on a non-frag_t is nonsense and will crash.
And a fragtree is a complete partition of the space.  What we really want
to check is if we know where to find the specific frag_t we need.

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph