ceph: move directory size logic to ceph_getattr

We can't fill i_size with rbytes at the fill_file_size stage without
adding additional checks for directories.  Notably, we want st_blocks
to remain 0 on directories so that 'du' still works.

Fill in i_blocks, i_size specially in ceph_getattr instead.

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