Revert Disable direct reclaim for z_wr_* threads

This commit used PF_MEMALLOC to prevent a memory reclaim deadlock.
However, commit 49be0ccf1fdc2ce852271d4d2f8b7a9c2c4be6db eliminated
the invocation of __cv_init(), which was the cause of the deadlock.
PF_MEMALLOC has the side effect of permitting pages from ZONE_DMA
to be allocated. The use of PF_MEMALLOC was found to cause stability
problems when doing swap on zvols. Since this technique is known to
cause problems and no longer fixes anything, we revert it.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #726