fs: btrfs: volumes: prevent overflow for multiplying

fs: btrfs: volumes: prevent overflow for multiplying

In __btrfs_map_block() we do a int * int and assign it to u64.
This is not safe as the result (int * int) is still evaluated as (int)
thus it can overflow.

Convert one of the multiplier to u64 to prevent such problem.

In real world, this should not cause problem as we have device number
limit thus it won't go beyond 4G for a single stripe.

But it's harder to teach coverity about all these hidden limits, so just
fix the possible overflow.

Reported-by: Coverity CID 312957
Reported-by: Coverity CID 312948
Signed-off-by: Qu Wenruo <wqu@suse.com>