libxfs: introduce libxfs_buf_read_uncached

libxfs: introduce libxfs_buf_read_uncached

Introduce an uncached read function so that userspace can handle them in
the same way as the kernel.  This also eliminates the need for some of
the libxfs_purgebuf calls (and two trips into the cache code).

Refactor the get/read uncached buffer functions to hide the details of
uncached buffer-ism in rdwr.c.

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>