9p: fix sparse warning: cast adds address space

Impact: Trust in the comment and add '__force' to the cast.

Fix this sparse warning:
  net/9p/trans_fd.c:420:34: warning: cast adds address space to expression (<asn:1>)

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>