ceph: make getxattr_cb return ssize_t

The getxattr_cb functions return size_t, which is unsigned and then
cast that value to int and then ssize_t before returning it. While all
of this works, it relies on implicit casting rules for signed/unsigned
conversions.

Change getxattr_cb to return ssize_t to better conform with what the
caller actually wants. Also, remove some suspicious casts.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Acked-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>