x86: use generic strncpy_from_user routine

The generic strncpy_from_user() is not really optimal, since it is
designed to work on both little-endian and big-endian.  And on
little-endian you can simplify much of the logic to find the first zero
byte, since little-endian arithmetic doesn't have to worry about the
carry bit propagating into earlier bytes (only later bytes, which we
don't care about).

But I have patches to make the generic routines use the architecture-
specific <asm/word-at-a-time.h> infrastructure, so that we can regain
the little-endian optimizations.  But before we do that, switch over to
the generic routines to make the patches each do just one well-defined
thing.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/uaccess.h

@@ -32,6 +32,7 @@
 
 #define segment_eq(a, b)	((a).seg == (b).seg)
 
+#define user_addr_max() (current_thread_info()->addr_limit.seg)
 #define __addr_ok(addr)					\
 	((unsigned long __force)(addr) <		\
 	 (current_thread_info()->addr_limit.seg))