ext4: fix ext4_tune_sb_params padding

The padding at the end of struct ext4_tune_sb_params is architecture
specific and in particular is different between x86-32 and x86-64,
since the __u64 member only enforces struct alignment on the latter.

This shows up as a new warning when test-building the headers with
-Wpadded:

include/linux/ext4.h:144:1: error: padding struct size to alignment boundary with 4 bytes [-Werror=padded]

All members inside the structure are naturally aligned, so the only
difference here is the amount of padding at the end. Make the padding
explicit, to have a consistent sizeof(struct ext4_tune_sb_params) of
232 on all architectures and avoid adding compat ioctl handling for
EXT4_IOC_GET_TUNE_SB_PARAM/EXT4_IOC_SET_TUNE_SB_PARAM.

This is an ABI break on x86-32 but hopefully this can go into 6.18.y early
enough as a fixup so no actual users will be affected.  Alternatively, the
kernel could handle the ioctl commands for both sizes (232 and 228 bytes)
on all architectures.

Fixes: 04a91570ac ("ext4: implemet new ioctls to set and get superblock parameters")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20251204101914.1037148-1-arnd@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

include/uapi/linux/ext4.h

@@ -139,7 +139,7 @@ struct ext4_tune_sb_params {
 	__u32 clear_feature_incompat_mask;
 	__u32 clear_feature_ro_compat_mask;
 	__u8  mount_opts[64];
-	__u8  pad[64];
+	__u8  pad[68];
 };
 
 #define EXT4_TUNE_FL_ERRORS_BEHAVIOR	0x00000001