BUG #18049: dynamic_shared_memory_type's value `posix` doesn't have any effect, syscall shm_get executes

The following bug has been logged on the website:

Bug reference:      18049
Logged by:          Jitender Kumar
Email address:      jkr0103@gmail.com
PostgreSQL version: 12.1
Operating system:   ubuntu 20.04
Description:

Expected `shm_open` syscall for the `posix` value of
`dynamic_shared_memory_type` parameter but strace shows `shm_get` being
executed.