ext4: ext4_xattr_value_same() should return false for external data

ext4_xattr_value_same() is used as a quick optimization in case the new
xattr value is identical to the previous value. When xattr value is
stored in a xattr inode the check becomes expensive so it is better to
just assume that they are not equal.

Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>