libxfs: use uncached buffers for initial mkfs writes

libxfs: use uncached buffers for initial mkfs writes

Teach mkfs to use uncached buffers to write the start and end of the
data device, the initial superblock, and the end of the realtime device
instead of open-coding uncached buffers.  This means we can get rid of
libxfs_purgebuf since we handle the state from the start now.

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>