Add MS_NOSYMFOLLOW for linux

[tedbrandston]

Looks like linux has had this since 5.10: https://www.phoronix.com/news/Linux-5.10-nosymfollow, and nix has it for bsd

At a glance it looks like all it takes is adding a comment and an MS_NOSYMFOLLOW to src/mount/linux.rs? Except maybe the libc crate doesn't expose it, yet?

[tedbrandston]

rust-lang/libc#4389 should add it to libc

[tedbrandston]

And once libc is updated: master...tedbrandston:nix:patch-1