LU-14597 flr: allow multiple primary mirrors

LU-14597 flr: allow multiple primary mirrors

Users can set "prefer" flag on any mirror/component, so the IO should
not report error if multiple mirrors are encountered.

Rename lod_mirror_entry::lme_primary to lme_prefer to avoid confusion.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I45748e56e38985a0d9028792ba3d976a4e03efb8
Reviewed-on: https://review.whamcloud.com/43247
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>