pNFS/flexfiles: Fix layoutcommit after a commit to DS

We should always do a layoutcommit after commit to DS, except if
the layout segment we're using has set FF_FLAGS_NO_LAYOUTCOMMIT.

Fixes: d67ae825 ("pnfs/flexfiles: Add the FlexFile Layout Driver")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>