exp/dir/filesystem: use packer to generate DirBlocks
exp/dir/filesystem: use packer to generate DirBlocks

The code here wasn't setting the DirEntry.Packdata field
because at one point that wasn't necessary for plain placking.
Then it became necessary, but everywhere else was using the packer
itself to set the field, so it got overlooked here.

Change-Id: I7c1550a994d07370047181ce53834cc4e6f252b5
Reviewed-on: https://upspin-review.googlesource.com/9560
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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