rxrpc: Fix untrusted unsigned subtract

In the Linux kernel, the following vulnerability has been resolved:

rxrpc: Fix untrusted unsigned subtract

Fix the following Smatch static checker warning:

net/rxrpc/rxgk_app.c:65 rxgk_yfs_decode_ticket() warn: untrusted unsigned subtract. 'ticket_len - 10 * 4'

by prechecking the length of what we're trying to extract in two places in the token and decoding for a response packet.

Also use sizeof() on the struct we're extracting rather specifying the size numerically to be consistent with the other related statements.