NFS: Fix filehandle bounds checking in nfs_fh_to_dentry()

[ Upstream commit ef93a685e01a281b5e2a25ce4e3428cf9371a205 ]

The function needs to check the minimal filehandle length before it can
access the embedded filehandle.

Reported-by: zhangjian <zhangjian496@huawei.com>
Fixes: 20fa1902 ("nfs: add export operations")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>