rxrpc: remove redundant initialization of variable 'len'

The variable 'len' is being initialized with a value that is never
read and it is re-assigned later, hence the initialization is redundant
and can be removed.

Cleans up clang warning:
net/rxrpc/recvmsg.c:275:15: warning: Value stored to 'len' during its
initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>