mds/Server: fix LOOKUPSNAP

mds/Server: fix LOOKUPSNAP

The current LOOKUPSNAP code path was dereferencing mdr->dn[0] and getting
SEGV.  Instead, set the do_lookup arg to false for getattr so that we do
not try to dereference this; for snaps it will always be NULL as we are
constructing a fake namespace for the .snap directory.  (This happens in
the path_traverse, the result of which is to set snapid--without any real
dentry).

With this fix, snaptest-0.sh passes.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>