sector, instead of failing the whole mount if it is garbage. Fields
in the fsinfo sector are only advisory, so there are better sanity
checks than this, and we already silently fix up the only other advisory
field in the fsinfo (the free cluster count).
This wasn't handled quite right in rev.1.92, 1.117, or in NetBSD. 1.92
also failed the whole mount for the non-garbage magic value 0xffffffff
1.117 fixed this well enough in practice since garbage values shouldn't
occur in practice, but left the error handling larger and more convoluted
than necessary. Now we handle the magic value as a special case of
fixing up all out of bounds values.
Also fix up the estimated next free cluster number when there is no
fsinfo sector. We were using 0, but CLUST_FIRST is safer.
Approved by: re (kensmith)
2e613b8127