libceph: fix osdmap decode error paths

Ensure that we set the err value correctly so that we do not pass a 0
value to ERR_PTR and confuse the calling code.  (In particular,
osd_client.c handle_map() will BUG(!newmap)).

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>
(cherry picked from commit 0ed7285e0001b960c888e5455ae982025210ed3d)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>