Detect kernels that honor gfp flags passed to vmalloc()

zfsonlinux/spl@2092cf68d8 used
PF_MEMALLOC to workaround a bug in the Linux kernel where
allocations did not honor the gfp flags passed to vmalloc().
Unfortunately, PF_MEMALLOC has the side effect of permitting
allocations to allocate pages outside of ZONE_NORMAL. This
has been observed to result in the depletion of ZONE_DMA32.

A kernel patch is available in the Gentoo bug tracker for
this issue.

  https://bugs.gentoo.org/show_bug.cgi?id=416685

This negates any benefit PF_MEMALLOC provides, so we introduce
an autotools check to disable the use of PF_MEMALLOC on
systems with patched kernels.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #126

module/spl/spl-kmem.c

@@ -843,6 +843,9 @@ kv_alloc(spl_kmem_cache_t *skc, int size, int flags)
 	if (skc->skc_flags & KMC_KMEM) {
 		ptr = (void *)__get_free_pages(flags, get_order(size));
 	} else {
+#ifdef HAVE_PMD_ALLOC_WITH_MASK
+		ptr = __vmalloc(size, flags|__GFP_HIGHMEM, PAGE_KERNEL);
+#else
 		/*
 		 * As part of vmalloc() an __pte_alloc_kernel() allocation
 		 * may occur.  This internal allocation does not honor the
@@ -866,6 +869,7 @@ kv_alloc(spl_kmem_cache_t *skc, int size, int flags)
 		} else {
 			ptr = __vmalloc(size, flags|__GFP_HIGHMEM, PAGE_KERNEL);
 		}
+#endif
 	}
 
 	/* Resulting allocated memory will be page aligned */