xfs: check sb_agblocks and sb_agblklog when validating superblock

xfs: check sb_agblocks and sb_agblklog when validating superblock

Source kernel commit: 4bb73d014785cc55225686f9f46e7192fb59d26b

Currently, we don't check sb_agblocks or sb_agblklog when we validate
the superblock, which means that we can fuzz garbage values into those
values and the mount succeeds.  This leads to all sorts of UBSAN
warnings in xfs/350 since we can then coerce other parts of xfs into
shifting by ridiculously large values.

Once we've validated agblocks, make sure the agcount makes sense.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
[sandeen: fix up u32 usage now so we keep building]
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>