f2fs: fix null-ptr-deref in f2fs_submit_page_bio()

There's issue as follows when concurrently installing the f2fs.ko
module and mounting the f2fs file system:
KASAN: null-ptr-deref in range [0x0000000000000020-0x0000000000000027]
RIP: 0010:__bio_alloc+0x2fb/0x6c0 [f2fs]
Call Trace:
<TASK>
f2fs_submit_page_bio+0x126/0x8b0 [f2fs]
__get_meta_page+0x1d4/0x920 [f2fs]
get_checkpoint_version.constprop.0+0x2b/0x3c0 [f2fs]
validate_checkpoint+0xac/0x290 [f2fs]
f2fs_get_valid_checkpoint+0x207/0x950 [f2fs]
f2fs_fill_super+0x1007/0x39b0 [f2fs]
mount_bdev+0x183/0x250
legacy_get_tree+0xf4/0x1e0
vfs_get_tree+0x88/0x340
do_new_mount+0x283/0x5e0
path_mount+0x2b2/0x15b0
__x64_sys_mount+0x1fe/0x270
do_syscall_64+0x5f/0x170
entry_SYSCALL_64_after_hwframe+0x76/0x7e

Above issue happens as the biset of the f2fs file system is not
initialized before register "f2fs_fs_type".
To address above issue just register "f2fs_fs_type" at the last in
init_f2fs_fs(). Ensure that all f2fs file system resources are
initialized.

Fixes: f543805fcd60 ("f2fs: introduce private bioset")
Signed-off-by: Ye Bin <[e-mail]>
Reviewed-by: Chao Yu <[e-mail]>
Signed-off-by: Jaegeuk Kim <[e-mail]>

fs/f2fs/super.c

···
 	err = f2fs_init_shrinker();
 	if (err)
 		goto free_sysfs;
-	err = register_filesystem(&f2fs_fs_type);
-	if (err)
-		goto free_shrinker;
 	f2fs_create_root_stats();
 	err = f2fs_init_post_read_processing();
 	if (err)
···
 	err = f2fs_create_casefold_cache();
 	if (err)
 		goto free_compress_cache;
+	err = register_filesystem(&f2fs_fs_type);
+	if (err)
+		goto free_casefold_cache;
 	return 0;
+free_casefold_cache:
+	f2fs_destroy_casefold_cache();
 free_compress_cache:
 	f2fs_destroy_compress_cache();
 free_compress_mempool:
···
 	f2fs_destroy_post_read_processing();
 free_root_stats:
 	f2fs_destroy_root_stats();
-	unregister_filesystem(&f2fs_fs_type);
-free_shrinker:
 	f2fs_exit_shrinker();
 free_sysfs:
 	f2fs_exit_sysfs();
···
 
 static void __exit exit_f2fs_fs(void)
 {
+	unregister_filesystem(&f2fs_fs_type);
 	f2fs_destroy_casefold_cache();
 	f2fs_destroy_compress_cache();
 	f2fs_destroy_compress_mempool();
···
 	f2fs_destroy_iostat_processing();
 	f2fs_destroy_post_read_processing();
 	f2fs_destroy_root_stats();
-	unregister_filesystem(&f2fs_fs_type);
 	f2fs_exit_shrinker();
 	f2fs_exit_sysfs();
 	f2fs_destroy_garbage_collection_cache();