libxfs: clean up libxfs_destroy

libxfs: clean up libxfs_destroy

It's weird that libxfs_init opens the three devices passed in via the
libxfs_xinit structure but libxfs_destroy doesn't actually close them.
Fix this inconsistency and remove all the open-coded device closing.

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