Revert Fix zpl_writepage() deadlock

The commit, cfc9a5c, to fix deadlocks
in zpl_writepage() relied on PF_MEMALLOC.   That had the effect of
disabling the direct reclaim path on all allocations originating from
calls to this function, but it failed to address the actual cause of
those deadlocks.  This led to the same deadlocks being observed with
swap on zvols, but not with swap on the loop device, which exercises
this code.

The use of PF_MEMALLOC also had the side effect of permitting
allocations to be made from ZONE_DMA in instances that did not require
it.  This contributes to the possibility of panics caused by depletion
of pages from ZONE_DMA.

As such, we revert this patch in favor of a proper fix for both issues.

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

module/zfs/zpl_file.c

@@ -358,20 +358,7 @@ zpl_putpage(struct page *pp, struct writeback_control *wbc, void *data)
 	ASSERT(PageLocked(pp));
 	ASSERT(!PageWriteback(pp));
 
-	/*
-	 * Disable the normal reclaim path for zpl_putpage().  This
-	 * ensures that all memory allocations under this call path
-	 * will never enter direct reclaim.  If this were to happen
-	 * the VM might try to write out additional pages by calling
-	 * zpl_putpage() again resulting in a deadlock.
-	 */
-	if (current->flags & PF_MEMALLOC) {
-		(void) zfs_putpage(mapping->host, pp, wbc);
-	} else {
-		current->flags |= PF_MEMALLOC;
-		(void) zfs_putpage(mapping->host, pp, wbc);
-		current->flags &= ~PF_MEMALLOC;
-	}
+	(void) zfs_putpage(mapping->host, pp, wbc);
 
 	return (0);
 }