btrfs: update superblock checksum on write

Note there are 32bytes on disk for the checksum, but only the first 4
are used. This is because btrfs can (or could, at some point?) use
sha256 instead of crc32 when higher reliability is needed (but high
performance isn't).

Change-Id: I8a2bcf8f462440568d9b3e2d9fbdb7208723bfb9
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1596
Reviewed-by: Chế Vũ Gia Hy <[email protected]>
Reviewed-by: Adrien Destugues <[email protected]>

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

@@ -66,7 +66,7 @@ static uint32 kCrcTable[256] = {
 	\param data Pointer to the byte stream.
 	\param length Length of the byte stream in bytes.
 
-	\return The crc checksum, or 0 if an error occurred.
+	\return The crc checksum.
 */
 uint32
 calculate_crc(uint32 crc, uint8* data, uint16 length)

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

@@ -15,6 +15,7 @@
 #include "BTree.h"
 #include "CachedBlock.h"
 #include "Chunk.h"
+#include "CRCTable.h"
 #include "DebugSupport.h"
 #include "ExtentAllocator.h"
 #include "Inode.h"
@@ -656,7 +657,14 @@ Volume::FindBlock(off_t logical, off_t& physical)
 status_t
 Volume::WriteSuperBlock()
 {
-	// TODO(lesderid): Calculate checksum
+	uint32 checksum = calculate_crc((uint32)~1,
+			(uint8 *)(&fSuperBlock + sizeof(fSuperBlock.checksum)),
+			sizeof(fSuperBlock) - sizeof(fSuperBlock.checksum));
+
+	fSuperBlock.checksum[0] = (checksum >>  0) & 0xFF;
+	fSuperBlock.checksum[1] = (checksum >>  8) & 0xFF;
+	fSuperBlock.checksum[2] = (checksum >> 16) & 0xFF;
+	fSuperBlock.checksum[3] = (checksum >> 24) & 0xFF;
 
 	if (write_pos(fDevice, BTRFS_SUPER_BLOCK_OFFSET, &fSuperBlock,
 			sizeof(btrfs_super_block))