KMC_KVMEM disrupts kv_alloc() memory alignment expectations

On kernels with KASAN enabled the following failure can be observed as
soon as the zfs module is loaded:

  VERIFY(IS_P2ALIGNED(ptr, PAGE_SIZE)) failed
  PANIC at spl-kmem-cache.c:228:kv_alloc()

The problem is kmalloc() has never guaranteed aligned allocations; this
requirement resulted in openzfs/spl@8b45dda which removed all
kmalloc() usage in kv_alloc().

Until a GFP_ALIGNED flag (or equivalent functionality) is provided by
the kernel this commit partially reverts 6695588 and 6d948c3 to
prevent k(v)malloc() allocations in kv_alloc().

Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #9813

module/os/linux/spl/spl-kmem-cache.c

@@ -202,26 +202,8 @@ kv_alloc(spl_kmem_cache_t *skc, int size, int flags)
 	if (skc->skc_flags & KMC_KMEM) {
 		ASSERT(ISP2(size));
 		ptr = (void *)__get_free_pages(lflags, get_order(size));
-	} else if (skc->skc_flags & KMC_KVMEM) {
-		ptr = spl_kvmalloc(size, lflags);
 	} else {
-		/*
-		 * GFP_KERNEL allocations can safely use kvmalloc which may
-		 * improve performance by avoiding a) high latency caused by
-		 * vmalloc's on-access allocation, b) performance loss due to
-		 * MMU memory address mapping and c) vmalloc locking overhead.
-		 * This has the side-effect that the slab statistics will
-		 * incorrectly report this as a vmem allocation, but that is
-		 * purely cosmetic.
-		 *
-		 * For non-GFP_KERNEL allocations we stick to __vmalloc.
-		 */
-		if ((lflags & GFP_KERNEL) == GFP_KERNEL) {
-			ptr = spl_kvmalloc(size, lflags);
-		} else {
-			ptr = __vmalloc(size, lflags | __GFP_HIGHMEM,
-			    PAGE_KERNEL);
-		}
+		ptr = __vmalloc(size, lflags | __GFP_HIGHMEM, PAGE_KERNEL);
 	}
 
 	/* Resulting allocated memory will be page aligned */
@@ -249,7 +231,7 @@ kv_free(spl_kmem_cache_t *skc, void *ptr, int size)
 		ASSERT(ISP2(size));
 		free_pages((unsigned long)ptr, get_order(size));
 	} else {
-		spl_kmem_free_impl(ptr, size);
+		vfree(ptr);
 	}
 }