btrfs: Fix pointer arithmetic in WriteSuperBlock()

The checksum was being calculated over the wrong memory region.
&fSuperBlock+N advances by N*sizeof(btrfs_super_block) bytes,
not N bytes. The correct form is (uint8*)&fSuperBlock+N which
advances by exactly N bytes past the start of the superblock,
correctly skipping the checksum field.

Change-Id: I8dba6410f2cd2d2d66e3cf484d4ac1f599449021
Reviewed-on: https://review.haiku-os.org/c/haiku/+/10447
Reviewed-by: Adrien Destugues <[email protected]>
Tested-by: Commit checker robot <[email protected]>

src/add-ons/kernel/file_systems/btrfs/Volume.cpp

@@ -510,7 +510,7 @@ status_t
 Volume::WriteSuperBlock()
 {
 	uint32 checksum = calculate_crc((uint32)~1,
-			(uint8 *)(&fSuperBlock + sizeof(fSuperBlock.checksum)),
+			(uint8 *)&fSuperBlock + sizeof(fSuperBlock.checksum),
 			sizeof(fSuperBlock) - sizeof(fSuperBlock.checksum));
 
 	fSuperBlock.checksum[0] = (checksum >>  0) & 0xFF;