Fix p0 initializer

Fix p0 initializer

Due to changes in the task_struct the following warning is occurs
when initializing the global p0.  Since this structure only exists
for it's address to be taken initialize it in a manor which isn't
sensitive to internal changes to the structure.

  module/spl/spl-generic.c:58:1: error: missing braces around
  initializer [-Werror=missing-braces]

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #576