fs/binfmt_elf.c: allocate initialized memory in fill_thread_core_info()

KMSAN reported uninitialized data being written to disk when dumping
core.  As a result, several kilobytes of kmalloc memory may be written
to the core file and then read by a non-privileged user.

Reported-by: sam <sunhaoyl@outlook.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20200419100848.63472-1-glider@google.com
Link: https://github.com/google/kmsan/issues/76
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
