f2fs: fix inline data not being written to disk in writeback path

f2fs: fix inline data not being written to disk in writeback path

When f2fs_fiemap() is called with `fileinfo->fi_flags` containing the
FIEMAP_FLAG_SYNC flag, it attempts to write data to disk before
retrieving file mappings via filemap_write_and_wait(). However, there is
an issue where the file does not get mapped as expected. The following
scenario can occur:

root@vm:/mnt/f2fs# dd if=/dev/zero of=data.3k bs=3k count=1
root@vm:/mnt/f2fs# xfs_io data.3k -c "fiemap -v 0 4096"
data.3k:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..5]:          0..5                 6 0x307

The root cause of this issue is that f2fs_write_single_data_page() only
calls f2fs_write_inline_data() to copy data from the data folio to the
inode folio, and it clears the dirty flag on the data folio. However, it
does not mark the data folio as writeback. When
__filemap_fdatawait_range() checks for folios with the writeback flag,
it returns early, causing f2fs_fiemap() to report that the file has no
mapping.

To fix this issue, the solution is to call
f2fs_write_single_node_folio() in f2fs_inline_data_fiemap() when
getting fiemap with FIEMAP_FLAG_SYNC flags. This patch ensures that the
inode folio is written back and the writeback process completes before
proceeding.

Cc: stable@kernel.org
Fixes: 9ffe0fb5f3bb ("f2fs: handle inline data operations")
Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>