libceph: rename ceph_msg::front_max to front_alloc_len

commit 3cea4c3071d4e55e9d7356efe9d0ebf92f0c2204 upstream.

Rename front_max field of struct ceph_msg to front_alloc_len to make
its purpose more clear.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>