linux: Implement vfs ioctl for inode flags set/get

[philmb3487_proton.me]

Summary

This is needed for some programs[1]. Since FreeBSD's vfs abstraction
does not understand fileattr's like Linux does, we'll stub it out.
Any attempt to set file attributes on the inode to anything else
than 0, results in an EINVAL.

1: cmake uses it to force case insensitivity if CASEFOLD is set.
Signed-off-by: Philippe Michaud-Boudreault <[e-mail]>