reactor: rewrite xfs_concurrency_from_kernel_version to use generic uname mechanism
reactor: rewrite xfs_concurrency_from_kernel_version to use generic uname mechanism

Since we have a generic mechanism, we can use it to reduce some custom code.

syuu1228/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar,dreamsxin/seastar