LU-12328 flr: preserve last read mirror

LU-12328 flr: preserve last read mirror

This patch preserves the mirror that has been read successfully
so that all subsequent I/O can take this advantage it and avoid
trying to read unavailable OSTs.

Signed-off-by: Jinshan Xiong <jinshan.xiong@gmail.com>
Change-Id: I806f936340db7c73228048edf21d5ecbed4b3c6c
Reviewed-on: https://review.whamcloud.com/35111
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>