LU-10503 flr: fix error handling in mirror_extend_file()

LU-10503 flr: fix error handling in mirror_extend_file()

This patch fixes the error handling issues in mirror_extend_file().

Change-Id: I388295886657cf9b9b072017002be937a9e657c0
Test-Parameters: trivial testlist=sanity-flr
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: https://review.whamcloud.com/30914
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Jenkins
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>