MFC r332042: Fix kernel memory disclosure in linux_ioctl_socket

strlcpy is used to copy a string into a buffer to be copied to userland,
previously leaving uninitialized data after the terminating NUL. Zero
the buffer first to avoid a kernel memory disclosure.

admbugs: 765, 811
Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reported by: Vlad Tsyrklevich
Sponsored by: The FreeBSD Foundation