attr: Fix reading of server_reply

read_server_reply first reads boothc header and then rest of packet
which contains hmac info. This should go in memory right after
boothc_header and not after full length of packet, because full length
of packet already contains hmac info.

Solution is to simply use length of header and not length of packet.

Longer term and better solution would be to drop read_server_reply
completely and use recv_auth which is used for everything else but attr
set and delete.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>