Fix --grow --add for linear arrays.

Fix --grow --add for linear arrays.

The new superblock needs to have a new disk.number.  This is a bit of a hack...
Fix handling of negative bitmap offsets on 64bit hosts.

The bitmap offset is a signed 32bit number, so casting to (long)
isn't sufficient.  We must cast to (int32_t).
Fix various problems with --grow --add for linear.

The code to add a drive to a live linear array had never
been tested properly and so was buggy.  This tidies it up
and means that the new regression-test passes.