os/FileStore: check replay guard on src for collection rename

This avoids a problematic sequence like:

     - rename A/ -> B/
     - remove B/1...100
     - destroy B/
     - create A/
     - write A/101...
     <crash>
     - replay A/ -> B/
     - remove B/1...100  (fails but tolerated)
     - destroy B/        (fails with ENOTEMPTY)

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