f2fs: avoid dirty node pages in check_only recovery

In the check_only mode, we should not make any dirty node pages. Otherwise,
we can get this panic:

F2FS-fs (nvme0n1p1): Need to recover fsync data
------------[ cut here ]------------
kernel BUG at fs/f2fs/node.c:2204!
CPU: 7 PID: 19923 Comm: mount Tainted: G           OE   4.9.8 #2
RIP: 0010:[<ffffffffc0979c0b>]  [<ffffffffc0979c0b>] flush_nat_entries+0x43b/0x7d0 [f2fs]
Call Trace:
 [<ffffffffc096ddaa>] ? __f2fs_submit_merged_bio+0x5a/0xd0 [f2fs]
 [<ffffffffc096ddaa>] ? __f2fs_submit_merged_bio+0x5a/0xd0 [f2fs]
 [<ffffffffc096dddb>] ? __f2fs_submit_merged_bio+0x8b/0xd0 [f2fs]
 [<ffffffff860e450f>] ? up_write+0x1f/0x40
 [<ffffffffc096dddb>] ? __f2fs_submit_merged_bio+0x8b/0xd0 [f2fs]
 [<ffffffffc0969f04>] write_checkpoint+0x2f4/0xf20 [f2fs]
 [<ffffffff860e938d>] ? trace_hardirqs_on+0xd/0x10
 [<ffffffffc0960bc9>] ? f2fs_sync_fs+0x79/0x190 [f2fs]
 [<ffffffffc0960bc9>] ? f2fs_sync_fs+0x79/0x190 [f2fs]
 [<ffffffffc0960bd5>] f2fs_sync_fs+0x85/0x190 [f2fs]
 [<ffffffffc097b6de>] f2fs_balance_fs_bg+0x7e/0x1c0 [f2fs]
 [<ffffffffc0977b64>] f2fs_write_node_pages+0x34/0x350 [f2fs]
 [<ffffffff860e5f42>] ? __lock_is_held+0x52/0x70
 [<ffffffff861d9b31>] do_writepages+0x21/0x30
 [<ffffffff86298ce1>] __writeback_single_inode+0x61/0x760
 [<ffffffff86909127>] ? _raw_spin_unlock+0x27/0x40
 [<ffffffff8629a735>] writeback_single_inode+0xd5/0x190
 [<ffffffff8629a889>] write_inode_now+0x99/0xc0
 [<ffffffff86283876>] iput+0x1f6/0x2c0
 [<ffffffffc0964b52>] f2fs_fill_super+0xc32/0x10c0 [f2fs]
 [<ffffffff86266462>] mount_bdev+0x182/0x1b0
 [<ffffffffc0963f20>] ? f2fs_commit_super+0x100/0x100 [f2fs]
 [<ffffffffc0960da5>] f2fs_mount+0x15/0x20 [f2fs]
 [<ffffffff86266e08>] mount_fs+0x38/0x170
 [<ffffffff86288bab>] vfs_kern_mount+0x6b/0x160
 [<ffffffff8628bcfe>] do_mount+0x1be/0xd60

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>