mds: scatter pin frozen tree on importer too

mds: scatter pin frozen tree on importer too

The importer also needs to scatter pin.  This avoids scatterlock gather
races like so:

A: start exporting to B
A: freeze, scatter pin tree
C: initiate gather
A: delay replay to gather
B: reply to gather, do not include (non-auth) dirfrag
A,B: finish migration
A: reply to gather, do not include (now non-auth) dirfrag
C: gets no info about the dirfrag!

By pinning on the importer, we ensure that at least one MDS will respond
to the gather with auth dirfrag info.

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