mm/maccess.c: actually return -EFAULT from strncpy_from_unsafe

As far as I can tell, strncpy_from_unsafe never returns -EFAULT.  ret is
the result of a __copy_from_user_inatomic(), which is 0 for success and
positive (in this case necessarily 1) for access error - it is never
negative.  So we were always returning the length of the, possibly
truncated, destination string.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/maccess.c

@@ -104,5 +104,5 @@ long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count)
 	pagefault_enable();
 	set_fs(old_fs);
 
-	return ret < 0 ? ret : src - unsafe_addr;
+	return ret ? -EFAULT : src - unsafe_addr;
 }