FROMGIT: f2fs: support to disable linear lookup fallback

After commit 91b587ba ("f2fs: Introduce linear search for
dentries"), f2fs forced to use linear lookup whenever a hash-based
lookup fails on casefolded directory, it may affect performance for
scenarios: a) create a new file w/ filename it doesn't exist in
directory, b) lookup a file which may be removed.

This patch supports to disable linear lookup fallback, so, once there is
a solution for commit 5c26d2f1 ("unicode: Don't special case
ignorable code points") to fix red heart unicode issue, then we can set
an encodeing flag to disable the fallback for performance recovery.

The way is kept in line w/ ext4, refer to commit 9e28059d ("ext4:
introduce linear search for dentries").

Bug: 410768629
(cherry picked from commit aa00c6d5
https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git dev)
Link: https://lore.kernel.org/linux-f2fs-devel/20250401035800.51504-1-chao@kernel.org


[Chao: backport dependent macro from commit 9e28059d ("ext4:
introduce linear search for dentries")]
Cc: Daniel Lee <chullee@google.com>
Cc: Gabriel Krisman Bertazi <krisman@suse.de>
Change-Id: Idff994f832dcaa1b9a5eb750b8ae03a5e580c1c6
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>