btrfs-progs: receive: better error reporting for snapshots

Two fixes:

1)

Check that the parent subvol actually is reachable via our root path.
The previous code wouldn't catch

    parent subvol: foo/bar
    root path:     bar   (i.e. mounted with -o subvol=bar)

where the parent isn't reachable from the root path.
(but the original "strstr(parent, root_path) == NULL" check still doesn't hold)

Also check for the slash after "root_path", i.e. throw an error on

    parent subvol: foobar
    root path:     foo

2)

If the parent subvol is the one that is mounted we obviously can't
receive into it, as it has to be read-only by definition.

We'd get a rather cryptic:

    At subvol /tmp/test/dest.snap
    At snapshot dest.snap
    ERROR: creating snapshot / -> dest.snap failed: Invalid cross-device link

(not sure what it says if "/" isn't even a btrfs)

But with this we get

    At subvol /tmp/test/dest.snap
    At snapshot dest.snap
    ERROR: creating snapshot . -> dest.snap failed: Read-only file system

which is both more helpful and more correct.

Signed-off-by: Benedikt Morbach <benedikt.morbach@googlemail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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