LU-13254 mdt: clear mti_mdt in mdt_thread_info_fini()

LU-13254 mdt: clear mti_mdt in mdt_thread_info_fini()

Clear mti_mdt when finalizing mdt_thread_info to prevent
its reuse my other handler later. Usually that may happen
at mdt_lvbo_fill/update() which takes thread info as is,
without initialization because at this point it is not
clear was it already initialized or not. So mti_mdt may be
used there being initialized by some other handler from
different MDT or even with garbage at old pointer.
Meanwhile there is no need to use any mdt_thread_info values
like mti_mdt in mdt_lvbo_fill() because there is MDT device
taken from namespace and the only fields are used from
mdt_thread_info are temporary storage for FID and lu_buffer.

Patch zeros mti_mdt upon thread finalizing and removes also
usages of info->mti_mdt from mdt_lvbo_fill/update() replacing
that with MDT taken from lock namespace.

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Ib350093f0b70c777932c056b34cb56a9702b650d
Reviewed-on: https://review.whamcloud.com/37592
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>