[patch] fix root opt for unionfs 2.x

[daniel at debian.org (Daniel Baumann)]

Jesse W. Hathaway wrote:
> nfsro is no longer an option in unionfs 2.0, if it
> is used the unioned volume attempts to write to the
> readonly nfs volume.

thanks, applied.

i bumped the check from -lt 19 to -lt 22; in debian, unionfs2 will be
used from 2.6.22 and newer.

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