13897 loader: implement mount/unmount rootfs

https://code.illumos.org/c/illumos-gate/+/1564

16822 EFI loader: Don't free bcache for DEVT_DISK devs

https://code.illumos.org/c/illumos-gate/+/3726

Those are logically related updates addressing the same problem, which is losing cached data (in loader block cache) as we repeatedly open and close files from the same rootfs and with every open, we need to re-read the file system metadata. The solution there is to keep rootfs “mounted”, that is, keep its metadata till we change  value of “currdev” variable.

Since we do not have integrated the metrics mechanism to make it possible to measure the speedup (referred in the issue #16822), we only can do limited measurements and need to rely on performance data posted with FreeBSD commits.

Thanks,

Toomas