f2fs: fix sbi->extent_list corruption issue

f2fs: fix sbi->extent_list corruption issue

When there is a failure in f2fs_fill_super() after/during
the recovery of fsync'd nodes, it frees the current sbi and
retries again. This time the mount is successful, but the files
that got recovered before retry, still holds the extent tree,
whose extent nodes list is corrupted since sbi and sbi->extent_list
is freed up. The list_del corruption issue is observed when the
file system is getting unmounted and when those recoverd files extent
node is being freed up in the below context.

list_del corruption. prev->next should be fffffff1e1ef5480, but was (null)
<...>
kernel BUG at kernel/msm-4.14/lib/list_debug.c:53!
lr : __list_del_entry_valid+0x94/0xb4
pc : __list_del_entry_valid+0x94/0xb4
<...>
Call trace:
__list_del_entry_valid+0x94/0xb4
__release_extent_node+0xb0/0x114
__free_extent_tree+0x58/0x7c
f2fs_shrink_extent_tree+0xdc/0x3b0
f2fs_leave_shrinker+0x28/0x7c
f2fs_put_super+0xfc/0x1e0
generic_shutdown_super+0x70/0xf4
kill_block_super+0x2c/0x5c
kill_f2fs_super+0x44/0x50
deactivate_locked_super+0x60/0x8c
deactivate_super+0x68/0x74
cleanup_mnt+0x40/0x78
__cleanup_mnt+0x1c/0x28
task_work_run+0x48/0xd0
do_notify_resume+0x678/0xe98
work_pending+0x8/0x14

Fix this by not creating extents for those recovered files if shrinker is
not registered yet. Once mount is successful and shrinker is registered,
those files can have extents again.

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>