efivars: introduce utf16_strncmp

Introduce utf16_strncmp which is used in the next patch.  Semantics
should be the same as the strncmp C function.

Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

drivers/firmware/efivars.c

@@ -169,6 +169,24 @@ utf16_strsize(efi_char16_t *data, unsigned long maxlength)
 	return utf16_strnlen(data, maxlength/sizeof(efi_char16_t)) * sizeof(efi_char16_t);
 }
 
+static inline int
+utf16_strncmp(const efi_char16_t *a, const efi_char16_t *b, size_t len)
+{
+	while (1) {
+		if (len == 0)
+			return 0;
+		if (*a < *b)
+			return -1;
+		if (*a > *b)
+			return 1;
+		if (*a == 0) /* implies *b == 0 */
+			return 0;
+		a++;
+		b++;
+		len--;
+	}
+}
+
 static efi_status_t
 get_var_data_locked(struct efivars *efivars, struct efi_variable *var)
 {