xdrgen: Fix opaque and string encoders for unbounded members

The variable-length opaque and string encoder templates emit an
unconditional bound check, "if (value->NAME.len > MAXSIZE) return
false". XDR represents an unbounded specifier (opaque foo<>, string
foo<>) as a maxsize of 0, so for an unbounded member the check
degenerates to "len > 0" and the generated encoder refuses every
non-empty value.

The decoder does not share this defect. It delegates to
xdrgen_decode_opaque() and xdrgen_decode_string(), which treat a
maxlen of 0 as unbounded and skip the length check. The sibling
variable-length array templates already guard their bound check
with maxsize != "0".

Guard the bound check the same way in each affected template -- the
struct and pointer forms of both the opaque and string encoders --
so an unbounded member encodes a payload of any length while a
bounded member keeps its limit.

An explicit zero-length bound (foo<0>) parses to the same maxsize of
0 and so also skips the check; xdrgen does not distinguish it from
the unbounded form, matching the decoder and the array encoders.

Fixes: 4b132aacb076 ("tools: Add xdrgen")
Link: https://patch.msgid.link/20260712193122.116845-6-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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