ext4, doc: fix incorrect h_reserved size

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.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

Documentation/filesystems/ext4/attributes.rst

@@ -76,7 +76,7 @@ The beginning of an extended attribute block is in
      - Checksum of the extended attribute block.
    * - 0x14
      - \_\_u32
-     - h\_reserved[2]
+     - h\_reserved[3]
      - Zero.
 
 The checksum is calculated against the FS UUID, the 64-bit block number