LU-14646 flr: write a FLR file downgrade SoM

LU-14646 flr: write a FLR file downgrade SoM

Seek over file size and write a FLR file does not change its SoM
and that makes file size incorrect.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I3075389721bdd40be60e9206c37f6c1bea514cce
Reviewed-on: https://review.whamcloud.com/43471
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>