xfs: check log iovec size to make sure it's plausibly a buffer log format

When log recovery is processing buffer log items, we should check that
the incoming iovec actually describes a region of memory large enough to
contain the log format and the dirty map.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>