Add missing __GFP_HIGHMEM flag to vmalloc

Make use of __GFP_HIGHMEM flag in vmem_alloc, which is required for
some 32-bit systems to make use of full available memory.
While kernel versions >=4.12-rc1 add this flag implicitly, older
kernels do not.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Closes openzfs#9031

module/spl/spl-kmem.c

@@ -180,7 +180,8 @@ spl_kmem_alloc_impl(size_t size, int flags, int node)
 		 */
 		if ((size > spl_kmem_alloc_max) || use_vmem) {
 			if (flags & KM_VMEM) {
-				ptr = __vmalloc(size, lflags, PAGE_KERNEL);
+				ptr = __vmalloc(size, lflags | __GFP_HIGHMEM,
+				    PAGE_KERNEL);
 			} else {
 				return (NULL);
 			}