arm64: tidy up Top-Byte-Ignore (TBI) in the kernel

In preparation for TBI to be enabled for processes from 15.0 we need
to clean up copying data between userspace and the kernel. These
functions will check the address is within the valid userspace range,
however as the userspace and kernel ranges may overlap when TBI is
enabled we need to mask off the top 8 bits.

Processes not using TBI are unaffected as the hardware will still
check all bits in the address, however this will happen at the first
load/store instruction.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D49119