linux: add trivial renameat2 implementation

[emaste]

Summary

Just return EINVAL if flags != 0. The Linux man page documents one case of EINVAL as "The filesystem does not support one of the flags in flags."

After rS351723 userland binaries will try using new system calls.

[emaste]

upload proper diff

[mjg]

sys/compat/linux/linux_file.c

731

I would add a rate check here to prevent spamming. You can find sample code in falloc_noinstall. It's literally few lines, but it's not a hard requirement.

[emaste]

Wait, this doesn't work.

[emaste]

make linux_renameat pass through to linux_renameat2

[emaste]

use c99 initializers suggested by mjg

[mjg]

sys/compat/linux/linux_file.c

694

}; should be on a new line