This fixes the below mem leak.

[  130.157600] =============================================================================
[  130.159662] BUG f2fs_page_array_entry-252:16 (Tainted: G        W  O     ): Objects remaining in f2fs_page_array_entry-252:16 on __kmem_cache_shutdown()
[  130.162742] -----------------------------------------------------------------------------
[  130.162742]
[  130.164979] Disabling lock debugging due to kernel taint
[  130.166188] INFO: Slab 0x000000009f5a52d2 objects=22 used=4 fp=0x00000000ba72c3e9 flags=0xfffffc0010200
[  130.168269] CPU: 7 PID: 3560 Comm: umount Tainted: G    B   W  O      5.9.0-rc4+ #35
[  130.170019] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
[  130.171941] Call Trace:
[  130.172528]  dump_stack+0x74/0x9a
[  130.173298]  slab_err+0xb7/0xdc
[  130.174044]  ? kernel_poison_pages+0xc0/0xc0
[  130.175065]  ? on_each_cpu_cond_mask+0x48/0x90
[  130.176096]  __kmem_cache_shutdown.cold+0x34/0x141
[  130.177190]  kmem_cache_destroy+0x59/0x100
[  130.178223]  f2fs_destroy_page_array_cache+0x15/0x20 [f2fs]
[  130.179527]  f2fs_put_super+0x1bc/0x380 [f2fs]
[  130.180538]  generic_shutdown_super+0x72/0x110
[  130.181547]  kill_block_super+0x27/0x50
[  130.182438]  kill_f2fs_super+0x76/0xe0 [f2fs]
[  130.183448]  deactivate_locked_super+0x3b/0x80
[  130.184456]  deactivate_super+0x3e/0x50
[  130.185363]  cleanup_mnt+0x109/0x160
[  130.186179]  __cleanup_mnt+0x12/0x20
[  130.187003]  task_work_run+0x70/0xb0
[  130.187841]  exit_to_user_mode_prepare+0x18f/0x1b0
[  130.188917]  syscall_exit_to_user_mode+0x31/0x170
[  130.189989]  do_syscall_64+0x45/0x90
[  130.190828]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  130.191986] RIP: 0033:0x7faf868ea2eb
[  130.192815] Code: 7b 0c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 90 f3 0f 1e fa 31 f6 e9 05 00 00 00 0f 1f 44 00 00 f3 0f 1e fa b8 a6 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 75 7b 0c 00 f7 d8 64 89 01
[  130.196872] RSP: 002b:00007fffb7edb478 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
[  130.198494] RAX: 0000000000000000 RBX: 00007faf86a18204 RCX: 00007faf868ea2eb
[  130.201021] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 000055971df71c50
[  130.203415] RBP: 000055971df71a40 R08: 0000000000000000 R09: 00007fffb7eda1f0
[  130.205772] R10: 00007faf86a04339 R11: 0000000000000246 R12: 000055971df71c50
[  130.208150] R13: 0000000000000000 R14: 000055971df71b38 R15: 0000000000000000
[  130.210515] INFO: Object 0x00000000a980843a @offset=744
[  130.212476] INFO: Allocated in page_array_alloc+0x3d/0xe0 [f2fs] age=1572 cpu=0 pid=3297
[  130.215030] 	__slab_alloc+0x20/0x40
[  130.216566] 	kmem_cache_alloc+0x2a0/0x2e0
[  130.218217] 	page_array_alloc+0x3d/0xe0 [f2fs]
[  130.219940] 	f2fs_init_compress_ctx+0x1f/0x40 [f2fs]
[  130.221736] 	f2fs_write_cache_pages+0x3db/0x860 [f2fs]
[  130.223591] 	f2fs_write_data_pages+0x2c9/0x300 [f2fs]
[  130.225414] 	do_writepages+0x43/0xd0
[  130.226907] 	__filemap_fdatawrite_range+0xd5/0x110
[  130.228632] 	filemap_write_and_wait_range+0x48/0xb0
[  130.230336] 	__generic_file_write_iter+0x18a/0x1d0
[  130.232035] 	f2fs_file_write_iter+0x226/0x550 [f2fs]
[  130.233737] 	new_sync_write+0x113/0x1a0
[  130.235204] 	vfs_write+0x1a6/0x200
[  130.236579] 	ksys_write+0x67/0xe0
[  130.237898] 	__x64_sys_write+0x1a/0x20
[  130.239309] 	do_syscall_64+0x38/0x90
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>