LU-12169 llite: fill copied dentry name's ending char properly

LU-12169 llite: fill copied dentry name's ending char properly

Dentry name expect an extra '\0'. and dentry_len won't calcualte
extra '\0' for it, but we should allocate memory and fill it
when copying dentry name by ourselves.

Otherwise, lu_name_is_valid_2() will try to access @name[len]
and check whether it is '\0'. this is invalid memory access.
We will possibly hit a crash if the first access that bit is '\0'.
and the bit overwritten by someone else, and finally we failed
sanity check in mdc_name_pack().

LustreError: 157839:0:(mdc_lib.c:137:mdc_pack_name()) LBUG

Fixes: f575b65("LU-12020 llite: make sure name pack atomic")
Change-Id: I533e19a0e6efb0fca5a46bcdbdb0006d1b1bedab
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/34611
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>