ext4: fix ext4_xattr_move_to_block()

When moving xattr entries from inline area to a xattr block, entries
that refer to external xattr inodes need special handling because
value data is not available in the inline area but rather should be
read from its external inode.

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