ext4, doc: fix incorrect h_reserved size

commit 7102ffe4 upstream.

According to document and code, ext4_xattr_header's size is 32 bytes, so
h_reserved size should be 3.

Signed-off-by: Wang Jianjian <wangjianjian3@huawei.com>
Link: https://lore.kernel.org/r/92fcc3a6-7d77-8c09-4126-377fcb4c46a5@huawei.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>