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").

Cc: Daniel Lee <chullee@google.com>
Cc: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>