md: Fix raid10 recovery problem.

md: Fix raid10 recovery problem.

There are two errors that can lead to recovery problems with raid10
when used in 'far' more (not the default).

Due to a '>' instead of '>=' the wrong block is located which would
result in garbage being written to some random location, quite
possible outside the range of the device, causing the newly
reconstructed device to fail.

The device size calculation had some rounding errors (it didn't round
when it should) and so recovery would go a few blocks too far which
would again cause a write to a random block address and probably
a device error.

The code for working with device sizes was fairly confused and spread
out, so this has been tided up a bit.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>