x86/mm: Stop printing BRK addresses

This currently leaks kernel physical addresses into userspace.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Link: https://lkml.kernel.org/r/20200229231120.1147527-1-nivedita@alum.mit.edu