The super block had a magic value that was used to identify that the
block should contain our data structure. But it was called an 'id'
which was confused with the header fsid in the past. Also, the btree
blocks aren't using a similar magic value at all.
This moves the magic value in to the header and creates values for the
super block and btree blocks. Both are written but the btree block
reads don't check the value.
Signed-off-by: Zach Brown <zab@versity.com>