LU-12020 llite: make sure name pack atomic

LU-12020 llite: make sure name pack atomic

We are trying to access dentry name directly and pass it
down without holding @d_lock, this is racy and possibly
make us trigger assertions:

(mdc_lib.c:137:mdc_pack_name()) ASSERTION( lu_name_is_valid_2(buf, cpy_len) ) failed:

Fix the problem by allocting memory and copy name with @d_lock
held.

Change-Id: Iae0066661f42e8fca9358cbedd9cb21828779bbb
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/34330
Tested-by: Jenkins
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>