ARM userspace accessors, e.g. {s,f}uword(9), copy{in,out}(9),
casuword(9) and others, use LDRT and STRT instructions to access
memory with the privileges of userspace. If the *RT instruction
faults on the kernel address, then additional checks must be done to
not confuse the VM system with invalid kernel-mode faults.

Put ARM on line with other FreeBSD architectures and disallow usermode
buffers which intersect with the kernel address space in advance,
before any accesses are performed. In other words, vm_fault(9) is no
longer called when e.g. suword(9) stores to invalid (i.e. not
userspace) address.

Also, switch ARM to use fueword(9) and casueword(9).

Note: there is a pending patch in D3617, which adds the special
processing for faults from LDRT and STRT. The addition of the
processing is useful for potential other uses of the instructions and
for completeness, but standard userspace accessors are better served
by not allowing such faults beforehand.

Reviewed by: andrew
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3816
MFC after: 2 weeks