nfsd: stricter decoding of write-like NFSv2/v3 ops

commit 13bf9fbff0e5e099e2b6f003a0ab8ae145436309 upstream.

The NFSv2/v3 code does not systematically check whether we decode past
the end of the buffer.  This generally appears to be harmless, but there
are a few places where we do arithmetic on the pointers involved and
don't account for the possibility that a length could be negative.  Add
checks to catch these.

Reported-by: Tuomas Haanpää <thaan@synopsys.com>
Reported-by: Ari Kauppi <ari@synopsys.com>
Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>