libceph: fix legacy layout decode with pool 0

If your data pool was pool 0, ceph_file_layout_from_legacy()
transform that to -1 unconditionally, which broke upgrades.
We only want do that for a fully zeroed ceph_file_layout,
so that it still maps to a file_layout_t.  If any fields
are set, though, we trust the fl_pgpool to be a valid pool.

Fixes: 7627151e ("libceph: define new ceph_file_layout structure")
Link: http://tracker.ceph.com/issues/17825


Signed-off-by: Yan, Zheng <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>