rc.d/linux: Attempt to mount only if necessary

[0mp]

Summary

Currently, if the linux service is run twice, mount fails with:

mount: linprocfs: Device busy
mount: linsysfs: Device busy
mount: devfs: Device busy
mount: fdescfs: Device busy
mount: tmpfs: Device busy

Additionally, the exit code of service linux start is 1 in such a case.

It is a bit more user-friendly if before running mount(8) the service
checks if there are any file systems let to be mounted.

Test Plan

The service still mounts file systems as before. Invoking service linux start for the second time no longer prints out error messages.

[trasz]

There is a somewhat related issue in https://reviews.freebsd.org/D26240. Do you think you could fix it as well, by throwing in an mkdir(1) somewhere in linux_mount?

[0mp]

In D32463#744552, @trasz wrote:

There is a somewhat related issue in https://reviews.freebsd.org/D26240. Do you think you could fix it as well, by throwing in an mkdir(1) somewhere in linux_mount?

Sure thing.

[0mp]

BTW, should we MFC this?

[trasz]

I think so, yeah. Please make sure to test it though; iirc previously the “nocover” option support wasn’t MFC-ed and I had to work around it.

[0mp]

I'll see to it when I have some time. If anyone wants to MFC that earlier than let me know or just commandeer it straight away.

[pauamma_gundo.com]

Noting to ease confusion that trhodes' commit was for D32436, not this.