linux_renameat2: improve flag checks

In the cases where Linux returns an error (e.g. passing in an undefined
flag) there's no need for us to emit a message. (The target of this
message is a developer working on the linuxulatorm, not the author of
presumably broken Linux software).

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21606