LU-14136 utils: correct error message in lfs mirror extend

LU-14136 utils: correct error message in lfs mirror extend

In migrate_open_files(), use llapi_layout_file_open() instead of
lfs_component_create() to avoid printing a confusing error message and
to ensure that the type of error is correctly propagated.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I31ebf29771b1a3e0b106a0a246a260a82eed92a4
Reviewed-on: https://review.whamcloud.com/40631
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>