[2.2.8 backport] config: fix dequeue_signal check for kernels <4.20

[behlendorf]

Motivation and Context

Issue #16858. Resolve Linux 4.19 kernel build failure.

Description

Before 4.20, kernel_siginfo_t was just called siginfo_t. This was causing the kthread_dequeue_signal_3arg_task check, which uses kernel_siginfo_t, to fail on older kernels.

In d6b8c17, we started checking for the "new" three-arg dequeue_signal() by testing for the "old" version. Because that test is explicitly using kernel_siginfo_t, it would fail, leading to the build trying to use the new three-arg version, which would then not comile.

This commit fixes that by avoiding checking for the old 3-arg dequeue_signal entirely. Instead, we check for the new one, as well as the 4-arg form, and we use the old form as a fallback. This way, we never have to test for it explicitly, and once we're building HAVE_SIGINFO will make sure we get the right kernel_siginfo_t for it, so everything works out nice.

How Has This Been Tested?

Clean cherry pick from master.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)