NFSv4.2: Always flush out writes in nfs42_proc_fallocate()

Whether we're allocating or delallocating space, we should flush out the
pending writes in order to avoid races with attribute updates.

Fixes: 1e564d3d ("NFSv4.2: Fix a race in nfs42_proc_deallocate()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>