Hide details of maximum user address calculation in a new
asm/uaccess.h interface named user_addr_max().

Provide little-endian implementation in find_zero(), which should work
but can probably be improved.

Abstrace alignment check behind IS_UNALIGNED() macro.

Kill double-semicolon, noticed by David Howells.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Compute a mask that will only have 0x80 in the bytes which
had a zero in them.  The formula is:

	~(((x & 0x7f7f7f7f) + 0x7f7f7f7f) | x | 0x7f7f7f7f)

In the inner word iteration, we have to compute the "x | 0x7f7f7f7f"
part, so we can reuse that in the above calculation.

Once we have this mask, we perform divide and conquer to find the
highest 0x80 location.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Linus removed the end-of-address-space hackery from
fs/namei.c:do_getname() so we really have to validate these edge
conditions and cannot cheat any more (as x86 used to as well).

Move to a common C implementation like x86 did.  And if both
src and dst are sufficiently aligned we'll do word at a time
copies and checks as well.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This mirrors x86 commit 9f0cf4ad
(x86: Use __builtin_object_size() to validate the buffer size for copy_from_user())
Signed-off-by: NDavid S. Miller <davem@davemloft.net>