Fix NULL deref in balance_pgdat()

Fix NULL deref in balance_pgdat()

Be careful not to unconditionally clear the PF_MEMALLOC bit in
the task structure.  It may have already been set when entering
kv_alloc() in which case it must remain set on exit.  In
particular the kswapd thread will have PF_MEMALLOC set in
order to prevent it from entering direct reclaim.  By clearing
it we allow the following NULL deref to potentially occur.

  BUG: unable to handle kernel NULL pointer dereference at (null)
  IP: [<ffffffff8109c7ab>] balance_pgdat+0x25b/0x4ff

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes ZFS issue #287