xfs_db: use uncached buffer reads to get the superblock

xfs_db: use uncached buffer reads to get the superblock

Upon startup, xfs_db needs to check if it is even looking at an XFS
filesystem, and it needs the AG 0 superblock contents to initialize the
incore mount.  We cannot know the filesystem sector size until we read
the superblock, but we also do not want to introduce aliasing in the
buffer cache.  Convert this code to the new uncached buffer read API so
that we can stop open-coding it.

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