LU-12944 mdd: pass correct xattr size to lower layers

LU-12944 mdd: pass correct xattr size to lower layers

In mdd_iterate_xattrs(), struct lu_buf allocated to store xattr value
can be reused for multiple xattrs, because it is only reallocated if
it happens to be too small for one xattr.
As a consequence, lb_len field does not represent actual xattr's size.
It has to be adjusted when passed to lower layers.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I26b54759b4e69fbac17a1032bbc724b796d78108
Reviewed-on: https://review.whamcloud.com/36689
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>