libibverbs: ignore IBV_FORK_SAFE et.al. when kernel has fork support

[krehm]

ibv_fork_init will now check if IBV_FORK_UNNEEDED is true, and if so will skip initialization of mm_root. Environment variables like IBV_FORK_SAFE and UCX_IB_FORK_INIT are silently ignored when the kernel has the necessary fork support.

[jgunthorpe]

@gal-pressman do you remember why we didn't do this from the start?

[gal-pressman]

https://github.com/linux-rdma/rdma-core/pull/975#issuecomment-824665681

[krehm]

In my case, it's not a question of performance improvement, it's a way to prevent segfaults when the user specifies an environment variable that enables ibv_fork_init. Background info available here.

[jgunthorpe]

Then it is probably best to move forward like this

[chien-intel]

Any issues moving forward with this PR?