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: f543805f ("f2fs: introduce private bioset")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>