NFSD: Fix ia_size underflow

[ Upstream commit e6faac3f58c7c4176b66f63def17a34232a17b0e ]

iattr::ia_size is a loff_t, which is a signed 64-bit type. NFSv3 and
NFSv4 both define file size as an unsigned 64-bit type. Thus there
is a range of valid file size values an NFS client can send that is
already larger than Linux can handle.

Currently decode_fattr4() dumps a full u64 value into ia_size. If
that value happens to be larger than S64_MAX, then ia_size
underflows. I'm about to fix up the NFSv3 behavior as well, so let's
catch the underflow in the common code path: nfsd_setattr().

Cc: stable@vger.kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
(cherry picked from commit e6faac3f58c7c4176b66f63def17a34232a17b0e)
[Larry: backport to 5.4.y. Minor conflict resolved due to missing commit 2f221d6f7b88
attr: handle idmapped mounts]
Signed-off-by: Larry Bassel <larry.bassel@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>