f2fs: fix livelock in swapfile writes

This patch fixes livelock in the below call path when writing swap pages.

[46374.617256] c2    701  __switch_to+0xe4/0x100
[46374.617265] c2    701  __schedule+0x80c/0xbc4
[46374.617273] c2    701  schedule+0x74/0x98
[46374.617281] c2    701  rwsem_down_read_failed+0x190/0x234
[46374.617291] c2    701  down_read+0x58/0x5c
[46374.617300] c2    701  f2fs_map_blocks+0x138/0x9a8
[46374.617310] c2    701  get_data_block_dio_write+0x74/0x104
[46374.617320] c2    701  __blockdev_direct_IO+0x1350/0x3930
[46374.617331] c2    701  f2fs_direct_IO+0x55c/0x8bc
[46374.617341] c2    701  __swap_writepage+0x1d0/0x3e8
[46374.617351] c2    701  swap_writepage+0x44/0x54
[46374.617360] c2    701  shrink_page_list+0x140/0xe80
[46374.617371] c2    701  shrink_inactive_list+0x510/0x918
[46374.617381] c2    701  shrink_node_memcg+0x2d4/0x804
[46374.617391] c2    701  shrink_node+0x10c/0x2f8
[46374.617400] c2    701  do_try_to_free_pages+0x178/0x38c
[46374.617410] c2    701  try_to_free_pages+0x348/0x4b8
[46374.617419] c2    701  __alloc_pages_nodemask+0x7f8/0x1014
[46374.617429] c2    701  pagecache_get_page+0x184/0x2cc
[46374.617438] c2    701  f2fs_new_node_page+0x60/0x41c
[46374.617449] c2    701  f2fs_new_inode_page+0x50/0x7c
[46374.617460] c2    701  f2fs_init_inode_metadata+0x128/0x530
[46374.617472] c2    701  f2fs_add_inline_entry+0x138/0xd64
[46374.617480] c2    701  f2fs_do_add_link+0xf4/0x178
[46374.617488] c2    701  f2fs_create+0x1e4/0x3ac
[46374.617497] c2    701  path_openat+0xdc0/0x1308
[46374.617507] c2    701  do_filp_open+0x78/0x124
[46374.617516] c2    701  do_sys_open+0x134/0x248
[46374.617525] c2    701  SyS_openat+0x14/0x20

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>