SCTP: Check to make sure file is valid before setting timeout

In-kernel sockets created with sock_create_kern don't usually
have a file and file descriptor allocated to them.  As a result,
when SCTP tries to check the non-blocking flag, we Oops when
dereferencing a NULL file pointer.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>