linux: Correct error handling for derive_psk_digest

[prashanth-nayak]

Treat only negative return values from derive_psk_digest () as errors.

[prashanth-nayak]

Without the fix:

ssan-rx2530-15:~ # nvme gen-tls-key -I 1 -m 1 -c nqn.1992-08.com.netapp:sn.0a86ed924b8d11efa459d039ea482fba:subsystem.s2 -i
NVMeTLSkey-1:01:BLRhgb8OqN3M2nchfHb1r8b0nUzOFe8KyGRGX56Jbbojq6oZ:
Inserted TLS key
00000000

With the fix:

ssan-rx2530-15:~ # nvme gen-tls-key -n nqn.2014-08.org.nvmexpress:uuid:713b3816-f9bf-ba43-b95a-5e4bf8c726e9 -c nqn.1992-08.com.netapp:sn.2f7701d3542f11efb872005056bba694:subsystem.ss1 -m 1 -I 1 -i
NVMeTLSkey-1:01:5ldrsnYzSywCj8FMdxV6kfLAMs/4torA6QhS8d9f0UTCGhPy:
Inserted TLS key 12daac64
ssan-rx2530-15:~ #

ssan-rx2530-15:~ # cat /proc/keys | grep NVMe
12daac64 I--Q--- 5 perm 3b010000 0 0 psk NVMe1R01 nqn.2014-08.org.nvmexpress:uuid:713b3816-f9bf-ba43-b95a-5e4bf8c726e9 nqn.1992-08.com.netapp:sn.2f7701d3542f11efb872005056bba694:subsystem.ss1 nQlz29o14678dui139NFoI7EeqVvpYsTlV7zQpdKdR4=: 32

[martin-gpy]

Thanks @prashanth-nayak

@igaw - This should fix #875

[igaw]

Thanks a lot!