Char: rocket, switch sleep_on to completion

rocket, switch sleep_on to completion

- sleep_on is deprecated and racy, use completion instead
- also check retval of interruptible function and return ERESTARTSYS
  eventually

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>