iscsi-scst: Add SHA256 and SHA3-256 support to CHAP

[bmeagherix]

At the moment SCST contains support for 2 CHAP algorithms: MD5 and SHA1. Linux LIO and open-iscsi both include these, as well as SHA256 and SHA3-256 (since 2019).

This PR adds SHA256 and SHA3-256 support to CHAP. Since in SCST CHAP negotiation is performed by the userspace iscsi-scstd daemon, the PR uses the kernel userspace AL_ALG API to access the additional hash functions.

If configured for CHAP, on iSCSI login the client will present an ordered list of desired algorithms. During this negotiation, if the client requests SHA256 or SHA3-256 we verify that the kernel supports the algorithm before agreeing to use it.

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Did some testing using open-iscsi with target settings modified from the default, e.g.

node.session.auth.chap_algs = SHA256
node.session.auth.username = testuser1
node.session.auth.password = testpass1234

And adding for Mutual Auth

node.session.auth.username_in = testuser2
node.session.auth.password_in = testpass4321

[lnocturno]

Hi Brian,

Thank you for the patch!

Thanks,
Gleb