mkfs.btrfs: fail on scandir error (-r mode)

mkfs.btrfs: fail on scandir error (-r mode)

mkfs.btrfs does not handle relative pathnames for now. When
they are passed to it it creates empty image. So first time
I thought it does not work at all.

This patch adds error handling for scandir(). With patch it behaves
this way:

    $ mkfs.btrfs -r ./root
    ...
    fs created label (null) on output.img
            nodesize 4096 leafsize 4096 sectorsize 4096 size 256.00MB
    Btrfs v0.19-52-g438c5ff-dirty
    scandir for ./root failed: No such file or directory
    unable to traverse_directory
    Making image is aborted.
    mkfs.btrfs: mkfs.c:1402: main: Assertion `!(ret)' failed.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Hugo Mills <hugo@carfax.org.uk>