f2fs: atomic: fix UAF issue on f2fs_inode_info.atomic_inode

f2fs: atomic: fix UAF issue on f2fs_inode_info.atomic_inode

- ioctl(F2FS_IOC_GARBAGE_COLLECT_RANGE)		- shrink
 - f2fs_gc
  - gc_data_segment
   - ra_data_block(cow_inode)
    - mapping = F2FS_I(inode)->atomic_inode->i_mapping
    : f2fs_is_cow_file(cow_inode) is true
						 - f2fs_evict_inode(atomic_inode)
						  - clear_inode_flag(fi->cow_inode, FI_COW_FILE)
						  - F2FS_I(fi->cow_inode)->atomic_inode = NULL
						  ...
						  - truncate_inode_pages_final(atomic_inode)
    - f2fs_grab_cache_folio(mapping)
    : create folio in atomic_inode->mapping
						  - clear_inode(atomic_inode)
						   - BUG_ON(atomic_inode->i_data.nrpages)

We need to add a reference on fi->atomic_inode before using its mapping
field during garbage collection, otherwise, it will cause UAF issue.

Cc: stable@kernel.org
Cc: Daeho Jeong <daehojeong@google.com>
Cc: Sunmin Jeong <s_min.jeong@samsung.com>
Fixes: 3db1de0e582c ("f2fs: change the current atomic write way")
Fixes: f18d00769336 ("f2fs: use meta inode for GC of COW file")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>