ceph: fix incorrect old_size length in ceph_mds_request_args

The 'old_size' is a __le64 type since birth, not sure why the
kclient incorrectly switched it to __le32. This change is okay
won't break anything because union will always allocate more memory
than the 'open' member needed.

Rename 'file_replication' to 'pool' as ceph did. Though this 'open'
struct may never be used in kclient in future, it's confusing when
going through the ceph code.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>