Fix kernel memory disclosure in svr4_sys_getdents64

svr4_sys_getdents64() copies a dirent structure to userland. When
calculating the record length for any given dirent entry alignment is
performed. However, the aligned bytes are not cleared, this will
trigger an info leak.

Direct commit to stable/11; this code does not exist in HEAD. This
change to be merged from stable/11 to stable/10.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
MFC after: 3 days
Security: Kernel memory disclosure (797)
Sponsored by: The FreeBSD Foundation