[uaccess] Rename userptr_sub() to userptr_diff()

[uaccess] Rename userptr_sub() to userptr_diff()

Clarify the intended usage of userptr_sub() by renaming it to
userptr_diff() (to avoid confusion with userptr_add()), and fix the
existing call sites that erroneously use userptr_sub() to subtract an
offset from a userptr_t value.

Signed-off-by: Michael Brown <mcb30@ipxe.org>