xtensa: add missing __user annotations to asm/uaccess.h

clear_user, strncpy_user, strnlen_user and their helpers operate on user
pointers, but don't have their arguments marked as __user.
Add __user annotation to userspace pointers of those functions.
Fix open-coded access check in the strnlen_user while at it.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>