libxfs: flush all dirty buffers and report errors when unmounting filesystem

libxfs: flush all dirty buffers and report errors when unmounting filesystem

Teach libxfs_umount to flush all dirty buffers when unmounting the
filesystem, to log write verifier errors and IO errors, and to return an
error code when things go wrong.  Subsequent patches will teach critical
utilities to exit with EXIT_FAILURE when this happens.

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