Patch: Add subvol current-generation command

Hey,

this is my first patch against something related to btrfs. The attached
patch adds the subvol current-generation command to retrieve the current
generation of the given subvol. This is inteded to be used with the
subvol find-new command.

After writing this patch I noticed that Sean Reifschneider already
sugested something similar in 2010 [1]. His patch reminded me of adding
a part to the manpage.

Thanks and Greetings
fabian

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[1] http://markmail.org/thread/iztosuxysjt4zx2b