new helper: uaccess_kernel()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/alpha/include/asm/uaccess.h

@@ -425,7 +425,7 @@ clear_user(void __user *to, long len)
 #undef __module_call
 
 #define user_addr_max() \
-        (segment_eq(get_fs(), USER_DS) ? TASK_SIZE : ~0UL)
+        (uaccess_kernel() ? ~0UL : TASK_SIZE)
 
 extern long strncpy_from_user(char *dest, const char __user *src, long count);
 extern __must_check long strlen_user(const char __user *str);