xtensa: rename PT_SIZE to PT_KERNEL_SIZE

PT_SIZE is used by the xtensa port to designate kernel exception frame
size. In preparation for struct pt_regs size change rename PT_SIZE to
PT_KERNEL_SIZE for clarity and change its definition to always cover
only the kernel exception frame.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>