LU-13918 lfs: let lfs mirror extend to use prefer flag

LU-13918 lfs: let lfs mirror extend to use prefer flag

so that lfs mirror extend -N --flags=prefer create new
mirror with prefer flag set and new IO go to the just
created mirror.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I2eb771fce3c14491eb193b053f3e97cc689b29c3
Reviewed-on: https://review.whamcloud.com/39698
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>