LU-15902 obdclass: dt_try_as_dir() check dir exists

LU-15902 obdclass: dt_try_as_dir() check dir exists

If an object is not directory, but dt_lookup() is called on it, it
may crash because .do_lookup is NULL for non-directory file.

Add argument to check object existence and type in dt_try_as_dir(),
and for object to create, skip this check.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I51df0cbb5a4e7abca370ee27dac678f995b76159
Reviewed-on: https://review.whamcloud.com/47483
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>