xfs_copy: don't hang if /all/ the targets hit write errors

xfs_copy: don't hang if /all/ the targets hit write errors

If xfs_copy is told to copy a filesystem and /all/ the writer threads
hit an write error, there won't be any threads to unlock mainwait, which
means that write_wbuf will deadlock with itself trying to lock mainwait.
Therefore, if we discover that all the writer threads are dead, just
bail out.

Discovered by running xfs/073 with a tiny test device.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>