KEYS: trusted: Do not use WARN when encode fails

In the Linux kernel, the following vulnerability has been resolved: KEYS: trusted: Do not use WARN when encode fails When asn1_encode_sequence() fails, WARN is not the correct solution. 1. asn1_encode_sequence() is not an internal function (located in lib/asn1_encode.c). 2. Location is known, which makes the stack trace useless. 3. Results a crash if panic_on_warn is set. It is also noteworthy that the use of WARN is undocumented, and it should be avoided unless there is a carefully considered rationale to use it. Replace WARN with pr_err, and print the return value instead, which is only useful piece of information.