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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>
Cc: [email protected]

Author: wangjianjian (C)

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