X.509: Introduce scope-based x509_certificate allocation

Add a DEFINE_FREE() clause for x509_certificate structs and use it in
x509_cert_parse() and x509_key_preparse().  These are the only functions
where scope-based x509_certificate allocation currently makes sense.
A third user will be introduced with the forthcoming SPDM library
(Security Protocol and Data Model) for PCI device authentication.

Unlike most other DEFINE_FREE() clauses, this one checks for IS_ERR()
instead of NULL before calling x509_free_certificate() at end of scope.
That's because the "constructor" of x509_certificate structs,
x509_cert_parse(), returns a valid pointer or an ERR_PTR(), but never
NULL.

Comparing the Assembler output before/after has shown they are identical,
save for the fact that gcc-12 always generates two return paths when
__cleanup() is used, one for the success case and one for the error case.

In x509_cert_parse(), add a hint for the compiler that kzalloc() never
returns an ERR_PTR().  Otherwise the compiler adds a gratuitous IS_ERR()
check on return.  Introduce an assume() macro for this which can be
re-used elsewhere in the kernel to provide hints for the compiler.

Suggested-by: Jonathan Cameron <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Link: https://lwn.net/Articles/934679/
Signed-off-by: Lukas Wunner <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>

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