fix FileSize to return correct disk usage recursively

[nonsense]

At the moment FileSize is not recursive and doesn't calculate correctly the disk usage of various paths.

This PR is fixing it, by recursively walking the path and adding together all file sizes.

---

Ultimately this is fixing a bug in the stores package, where we iterate over all storage reservations and then check how much disk space is used per sector:

	for id, ft := range p.reservations {
		for _, fileType := range storiface.PathTypes {
			if fileType&ft == 0 {
				continue
			}

			sp := p.sectorPath(id, fileType)
			used, err := ls.DiskUsage(sp)

sp in this case has the form of ./cache/s-t0127896-36 which is actually a directory with many .dat files, and not a single file.

---

TODO:

• test with lotus-miner

[nonsense]

I think this should be correct now. On my miner all the Reserved space is now gone as expected:

before

9c92d406-1e0b-4edd-93ba-1f413ca5431d:
        [####################################**************] 1.789 TiB/1.789 TiB 100%
        Unsealed: 2; Sealed: 1; Caches: 1; Reserved: 483.2 GiB
        Weight: 10; Use: Seal
        Local: /root/storage_sealing
        URL: http://127.0.0.1:2345/remote

after

9c92d406-1e0b-4edd-93ba-1f413ca5431d:
        [##########################################        ] 1.51 TiB/1.789 TiB 84%
        Unsealed: 2; Sealed: 2; Caches: 2; Reserved: 0 B
        Weight: 10; Use: Seal
        Local: /root/storage_sealing
        URL: http://127.0.0.1:2345/remote

(both before and after keep 2 sectors in the sealing directory)

[nonsense]

Given that this is a _unix.go file, I think we must always have a value in Sys(), so I don't think we need to check for nil pointer here.

[magik6k]

Closes #3124

[magik6k]

(LGTM)