rxrpc: rxkad: Remove redundant variable offset

Variable offset is being assigned a value from a calculation
however the variable is never read, so this redundant variable
can be removed.

Cleans up the following clang-analyzer warning:

net/rxrpc/rxkad.c:579:2: warning: Value stored to 'offset' is never read
[clang-analyzer-deadcode.DeadStores].

net/rxrpc/rxkad.c:485:2: warning: Value stored to 'offset' is never read
[clang-analyzer-deadcode.DeadStores].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>