fork: Define usercopy region in mm_struct slab caches

In support of usercopy hardening, this patch defines a region in the
mm_struct slab caches in which userspace copy operations are allowed.
Only the auxv field is copied to userspace.

cache object allocation:
    kernel/fork.c:
        #define allocate_mm()     (kmem_cache_alloc(mm_cachep, GFP_KERNEL))

        dup_mm():
            ...
            mm = allocate_mm();

        copy_mm(...):
            ...
            dup_mm();

        copy_process(...):
            ...
            copy_mm(...)

        _do_fork(...):
            ...
            copy_process(...)

example usage trace:

    fs/binfmt_elf.c:
        create_elf_tables(...):
            ...
            elf_info = (elf_addr_t *)current->mm->saved_auxv;
            ...
            copy_to_user(..., elf_info, ei_index * sizeof(elf_addr_t))

        load_elf_binary(...):
            ...
            create_elf_tables(...);

This region is known as the slab cache's usercopy region. Slab caches
can now check that each dynamically sized copy operation involving
cache-managed memory falls entirely within the slab's usercopy region.

This patch is modified from Brad Spengler/PaX Team's PAX_USERCOPY
whitelisting code in the last public patch of grsecurity/PaX based on my
understanding of the code. Changes or omissions from the original code are
mine and don't reflect the original grsecurity/PaX code.

Signed-off-by: David Windsor <dave@nullcore.net>
[kees: adjust commit log, split patch, provide usage trace]
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Rik van Riel <riel@redhat.com>