KEYS: trusted: dcp: fix key_len validation and calc_blob_len() return type

Two correctness and type-hygiene issues exist in the DCP trusted keys
implementation.

First, trusted_dcp_unseal() reads p->key_len from a user-supplied blob
without checking if it exceeds MAX_KEY_SIZE.  If a crafted blob provides a
payload_len larger than 128, the subsequent do_aead_crypto() call writes
past the end of the p->key array into the adjacent p->blob buffer within
the same struct trusted_key_payload -- the caller's own input, not
unrelated kernel memory.  While not exploitable, this violates strict array
bounds and triggers static analyzers.  Fix this by adding a validation
check against MIN_KEY_SIZE and MAX_KEY_SIZE immediately after reading the
length, matching the checks already done in trusted_core.c.

Second, calc_blob_len() calculates a sum in size_t that truncates to
unsigned int on 64-bit platforms.  Because the DCP hardware is only present
on 32-bit i.MX SoC platforms, size_t and unsigned int are functionally
equivalent in production, making this truncation harmless in practice.
Nevertheless, updating the return type to size_t (and subsequently updating
'blen' in the seal/unseal paths) resolves type-narrowing warnings and
improves overall code hygiene.

Fixes: 2e8a0f40a39c ("KEYS: trusted: Introduce NXP DCP-backed trusted keys")
Signed-off-by: Fabrice Derepas <fabrice.derepas@canonical.com>
Reviewed-by: David Gstir <david@sigma-star.at>
Reviewed-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20260719163939.3624767-1-fabrice.derepas@canonical.com
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

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