net/9p: Switch to wait_event_killable()

net/9p: Switch to wait_event_killable()

Because userspace gets Very Unhappy when calls like stat() and execve()
return -EINTR on 9p filesystem mounts. For instance, when bash is
looking in PATH for things to execute and some SIGCHLD interrupts
stat(), bash can throw a spurious 'command not found' since it doesn't
retry the stat().

In practice, hitting the problem is rare and needs a really
slow/bogged down 9p server.

Cc: [email protected]
Signed-off-by: Tuomas Tynkkynen <[email protected]>
Signed-off-by: Al Viro <[email protected]>