LU-14119 lfsck: replace dt_lookup() with dt_lookup_dir()

LU-14119 lfsck: replace dt_lookup() with dt_lookup_dir()

Lfsck code calls dt_lookup() to lookup sub file under directory in
many places, but this function needs to to initialize directory with
dt_try_as_dir() first, while it's missing in several places, since
the overhead is trivial, call dt_lookup_dir() instead.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I40bd8d51edece50353af1729cf867572a0abea78