LU-9771 flr: support extending a plain file to mirrored file

LU-9771 flr: support extending a plain file to mirrored file

Support extending a plain file to mirrored file by
'lfs mirror extend -N <a_plain_file>', so that <a_plain_file>
becomes a mirrored file, and it contins 2 mirrors each of them
has its old layout parameters covering [0, EOF).

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I415cd65effa72a4bde70b02063d8b635866947a8
Reviewed-on: https://review.whamcloud.com/29665
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>