Btrfs: allow running defrag in parallel to administrative tasks

Commit 5ac00add added a testnset mutex and code that disallows
running administrative tasks in parallel. It is prevented that
the device add/delete/balance/replace/resize operations are
started in parallel. By mistake, the defragmentation operation
was included in the check for mutually exclusiveness as well.
This is fixed with this commit.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>