BACKPORT: FROMGIT: f2fs: add sysfs entry for effective lookup mode

This commit introduces a new read-only sysfs entry at
/sys/fs/f2fs/<device>/effective_lookup_mode.

This entry displays the actual directory lookup mode F2FS is
currently using. This is needed for debugging and verification,
as the behavior is determined by both on-disk flags and mount
options.

Bug: 439513881
Bug: 432807936
Bug: 441173933
Bug: 443659718
(cherry picked from commit 1bd119da
 https: //git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git dev)
Link: https://lore.kernel.org/linux-f2fs-devel/20250805065228.1473089-1-chullee@google.com/


Change-Id: Iabb9548bb9f73ef42d46ed0506a0372ec37e75d8
[chullee: resolved merge conflicts in the document]
Signed-off-by: Daniel Lee <chullee@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 8c91b368)