LU-10070 lod: SEL: Add flag & setstripe support

LU-10070 lod: SEL: Add flag & setstripe support

The self-extending layouts feature adds a new layout flag
and also uses the stripe size field differently.

This patch implements this basic functionality, to be used
in subsequent patches.

Cray-bug-id: LUS-2528
Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: I4392b70266cbab5bc8fa42afc3c360b954d5918a
Reviewed-on: https://review.whamcloud.com/33782
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>