btrfs: store precalculated csum_size in fs_info

In many places we need the checksum size and it is inefficient to read
it from the raw superblock. Store the value into fs_info, actual use
will be in followup patches.  The size is u32 as it allows to generate
better assembly than with u16.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -937,6 +937,7 @@ struct btrfs_fs_info {
 	u32 sectorsize;
 	/* ilog2 of sectorsize, use to avoid 64bit division */
 	u32 sectorsize_bits;
+	u32 csum_size;
 	u32 stripesize;
 
 	/* Block groups and devices containing active swapfiles. */

fs/btrfs/disk-io.c

@@ -3080,6 +3080,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 	fs_info->nodesize = nodesize;
 	fs_info->sectorsize = sectorsize;
 	fs_info->sectorsize_bits = ilog2(sectorsize);
+	fs_info->csum_size = btrfs_super_csum_size(disk_super);
 	fs_info->stripesize = stripesize;
 
 	/*