mkfs: create initial subvolume

[kdave]

Create an initial subvolume or subvolumes.
Usecase:

• allow --root-dir to put the files there
• allow creating nested subvolumes
• allow creating arbitrary paths
• allow multiple subvolumes

[kdave]

https://patchwork.kernel.org/patch/9930323/ + other patches in the mailingist

[kdave]

Initial support added to devel, --rootdir + --subvol will fill the given subvolume. The final semantics will be something like this:

Sounds good, I think we can achieve more than just subvol + copy of a
directory, with a extension of the command line ui. This should allow to
create multiple subvolumes, nested (or not) with contents of the
directory copied over any pre-created contents from --rootdir.

First, the option --subvol could be specified repeatedly. Each path
component that would not exist will become a directory, the last one
will be subvolume.

With --rootdir among the --subvol options, all existing paths would be
reused (as long files and directories/subvolumes don't clash).

Example, immitating one of used root filesystem layout:

expected result
/
/@ (subvolume
/@/boot (subvolume)
/@/home (subvolume)
/@/var/log (subvolume)
/@/tmp (subvolume)

and over that, same as if some tarball is unpacked, creating /bin,
/etc, /usr, /sbin etc.

would be generated by by something like:

$ mkfs.btrfs -S /@ -S /@/boot -S /@/home -S /@/var/log -S /@/tmp --rootdir rootfiles/ /path/to/device

Reusing --rootdir might be a bad idea, so we'd have add a new option
that would specify the target path and the source directory.