f2fs: use kvmalloc, if kmalloc is failed

One report says memalloc failure during mount.

 (unwind_backtrace) from [<c010cd4c>] (show_stack+0x10/0x14)
 (show_stack) from [<c049c6b8>] (dump_stack+0x8c/0xa0)
 (dump_stack) from [<c024fcf0>] (warn_alloc+0xc4/0x160)
 (warn_alloc) from [<c0250218>] (__alloc_pages_nodemask+0x3f4/0x10d0)
 (__alloc_pages_nodemask) from [<c0270450>] (kmalloc_order_trace+0x2c/0x120)
 (kmalloc_order_trace) from [<c03fa748>] (build_node_manager+0x35c/0x688)
 (build_node_manager) from [<c03de494>] (f2fs_fill_super+0xf0c/0x16cc)
 (f2fs_fill_super) from [<c02a5864>] (mount_bdev+0x15c/0x188)
 (mount_bdev) from [<c03da624>] (f2fs_mount+0x18/0x20)
 (f2fs_mount) from [<c02a68b8>] (mount_fs+0x158/0x19c)
 (mount_fs) from [<c02c3c9c>] (vfs_kern_mount+0x78/0x134)
 (vfs_kern_mount) from [<c02c76ac>] (do_mount+0x474/0xca4)
 (do_mount) from [<c02c8264>] (SyS_mount+0x94/0xbc)
 (SyS_mount) from [<c0108180>] (ret_fast_syscall+0x0/0x48)

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index 666518e..aed7084 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -142,7 +142,7 @@ int f2fs_start_gc_thread(struct f2fs_sb_info *sbi)
 			"f2fs_gc-%u:%u", MAJOR(dev), MINOR(dev));
 	if (IS_ERR(gc_th->f2fs_gc_task)) {
 		err = PTR_ERR(gc_th->f2fs_gc_task);
-		kfree(gc_th);
+		kvfree(gc_th);
 		sbi->gc_thread = NULL;
 	}
 out:
@@ -155,7 +155,7 @@ void f2fs_stop_gc_thread(struct f2fs_sb_info *sbi)
 	if (!gc_th)
 		return;
 	kthread_stop(gc_th->f2fs_gc_task);
-	kfree(gc_th);
+	kvfree(gc_th);
 	sbi->gc_thread = NULL;
 }