fsck.f2fs: support to tune linear lookup feature

Support a new option --nolinear-lookup=X for fsck.f2fs to tune
linear lookup fallback conditionally, X=1: disable linear lookup,
X=0: enable linear lookup.

This can help to 1) add a regression testcase to check kernel
whether linear lookup fallback has fixed unicode red heart lookup
issue, 2) once unicode bug has been fixed, we can use this option
to disable linear lookup for performance recovery.

Cc: Daniel Lee <chullee@google.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>