Fix Large kmem_alloc in vdev_metaslab_init

This allocation can go way over 1MB, so we should use vmem_alloc
instead of kmem_alloc.

  Large kmem_alloc(1430784, 0x1000), please file an issue...
  Call Trace:
   [<ffffffffa0324aff>] ? spl_kmem_zalloc+0xef/0x160 [spl]
   [<ffffffffa17d0c8d>] ? vdev_metaslab_init+0x9d/0x1f0 [zfs]
   [<ffffffffa17d46d0>] ? vdev_load+0xc0/0xd0 [zfs]
   [<ffffffffa17d4643>] ? vdev_load+0x33/0xd0 [zfs]
   [<ffffffffa17c0004>] ? spa_load+0xfc4/0x1b60 [zfs]
   [<ffffffffa17c1838>] ? spa_tryimport+0x98/0x430 [zfs]
   [<ffffffffa17f28b1>] ? zfs_ioc_pool_tryimport+0x41/0x80 [zfs]
   [<ffffffffa17f5669>] ? zfsdev_ioctl+0x4a9/0x4e0 [zfs]
   [<ffffffff811bacdf>] ? do_vfs_ioctl+0x2cf/0x4b0
   [<ffffffff811baf41>] ? SyS_ioctl+0x81/0xa0

Signed-off-by: Chunwei Chen <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #4752

module/zfs/vdev.c

@@ -892,11 +892,11 @@ vdev_metaslab_init(vdev_t *vd, uint64_t txg)
 
 	ASSERT(oldc <= newc);
 
-	mspp = kmem_zalloc(newc * sizeof (*mspp), KM_SLEEP);
+	mspp = vmem_zalloc(newc * sizeof (*mspp), KM_SLEEP);
 
 	if (oldc != 0) {
 		bcopy(vd->vdev_ms, mspp, oldc * sizeof (*mspp));
-		kmem_free(vd->vdev_ms, oldc * sizeof (*mspp));
+		vmem_free(vd->vdev_ms, oldc * sizeof (*mspp));
 	}
 
 	vd->vdev_ms = mspp;
@@ -950,7 +950,7 @@ vdev_metaslab_fini(vdev_t *vd)
 			if (msp != NULL)
 				metaslab_fini(msp);
 		}
-		kmem_free(vd->vdev_ms, count * sizeof (metaslab_t *));
+		vmem_free(vd->vdev_ms, count * sizeof (metaslab_t *));
 		vd->vdev_ms = NULL;
 	}