[ARM] fix a couple clear_user_highpage assembly constraints

In all cases the kaddr is assigned an input register even though it is
modified in the assembly code.  Let's assign a new variable to the
modified value and mark those inline asm with volatile otherwise they
get optimized away because the output variable is otherwise not used.

Also fix a few conversion errors in copypage-feroceon.c and
copypage-v4mc.c.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>