Don't set NFSv4 ACL inheritance flags on non-directories.

They only make sense in the context of directory ACLs, and attempting
to set them on regular files results in errors, causing a recursive
setfacl invocation to abort.

This is derived from patches by Shawn Webb <shawn.webb@hardenedbsd.org>
and Mitchell Horne <mhorne063@gmail.com>.

PR: 155163
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D15061