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

stable inclusion
from linux-4.19.191
commit e5370bd9e419fcac4f8cf7b242455ba121212037

--------------------------------

[ Upstream commit 99f23783 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>