Error handling in C callbacks

[robin-nitrokey]

Context: #124

We should return an error in these cases:

• conversion from u32 to usize fails
• offset calculation overflows
• block exceeds the configured block count

I think LFS_ERR_INVAL would be an appropriate error code.

In a second step, we should make sure that overflows cannot happen by validating the lengths in Allocation::with_config.

@sosthene-nitrokey What do you think?