xfs_copy: use uncached buffer reads to get the superblock

xfs_copy: use uncached buffer reads to get the superblock

Upon startup, xfs_copy needs to read the filesystem superblock to mount
the filesystem.  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>