The immediate write mirroring flag is a file level layout flag, like the FLR state flags (eg, LCM_FL_RDONLY).  Currently it is not possible to set these flags from userspace, but we want to be able to add them when we update a file.

Because this flag change affects ongoing operations, it will have to be a layout intent, with handling similar to lfs mirror resync.

Specifically, we'll need to take the layout to SYNC_PENDING, flush all writes, then make the change conditional on the layout being in SYNC_PENDING.  This will be really very similar to that case.