ext4: zero out inline data using memset() instead of empty_zero_page

Not all architectures (in particular, sparc64) have empty_zero_page.
So instead of copying from empty_zero_page, use memset to clear the
inline data by signalling to ext4_xattr_set_entry() via a magic
pointer value, EXT4_ZERO_ATTR_VALUE, which is defined by casting -1 to
a pointer.

This fixes a build failure on sparc64, and the memset() should be more
efficient than using memcpy() anyway.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/xattr.h b/fs/ext4/xattr.h
index 7b5513e..69eda78 100644
--- a/fs/ext4/xattr.h
+++ b/fs/ext4/xattr.h
@@ -71,6 +71,7 @@
 #define BFIRST(bh) ENTRY(BHDR(bh)+1)
 #define IS_LAST_ENTRY(entry) (*(__u32 *)(entry) == 0)
 
+#define EXT4_ZERO_XATTR_VALUE ((void *)-1)
 
 struct ext4_xattr_info {
 	int name_index;