rocket.c: fix unchecked mutex_lock_interruptible()

Check the return of mutex_lock_interruptible() in drivers/char/rocket.c and
return ERESTARTSYS if we were interrupted.

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