loader: Don't free bcache for devices of type DEVT_DISK

[cperciva]

Summary

In my testing, this speeds up the loader from 513 ms to 474 ms, reduces the number of I/Os performed from 609 to 432, and reduces the total number of blocks read from 61963 to 60797.

[cperciva]

This solves the "calling mount doesn't actually hold the disk open if there's no filesystem to mount" problem.

[imp]

Seems good to my eye.

[tsoome]

The obvious flaw is about possible unfreed memory, but that shouldn't be big problem anyhow.