nfsd: inherit required unset default acls from effective set

A well-formed NFSv4 ACL will always contain OWNER@/GROUP@/EVERYONE@
ACEs, but there is no requirement for inheritable entries for those
entities. POSIX ACLs must always have owner/group/other entries, even for a
default ACL.

nfsd builds the default ACL from inheritable ACEs, but the current code
just leaves any unspecified ACEs zeroed out. The result is that adding a
default user or group ACE to an inode can leave it with unwanted deny
entries.

For instance, a newly created directory with no acl will look something
like this:

	# NFSv4 translation by server
	A::OWNER@:rwaDxtTcCy
	A::GROUP@:rxtcy
	A::EVERYONE@:rxtcy

	# POSIX ACL of underlying file
	user::rwx
	group::r-x
	other::r-x

...if I then add new v4 ACE:

	nfs4_setfacl -a A:fd:1000:rwx /mnt/local/test

...I end up with a result like this today:

	user::rwx
	user:1000:rwx
	group::r-x
	mask::rwx
	other::r-x
	default:user::---
	default:user:1000:rwx
	default:group::---
	default:mask::rwx
	default:other::---

	A::OWNER@:rwaDxtTcCy
	A::1000:rwaDxtcy
	A::GROUP@:rxtcy
	A::EVERYONE@:rxtcy
	D:fdi:OWNER@:rwaDx
	A:fdi:OWNER@:tTcCy
	A:fdi:1000:rwaDxtcy
	A:fdi:GROUP@:tcy
	A:fdi:EVERYONE@:tcy

...which is not at all expected. Adding a single inheritable allow ACE
should not result in everyone else losing access.

The setfacl command solves a silimar issue by copying owner/group/other
entries from the effective ACL when none of them are set:

    "If a Default ACL entry is created, and the  Default  ACL  contains  no
     owner,  owning group,  or  others  entry,  a  copy of the ACL owner,
     owning group, or others entry is added to the Default ACL.

Having nfsd do the same provides a more sane result (with no deny ACEs
in the resulting set):

	user::rwx
	user:1000:rwx
	group::r-x
	mask::rwx
	other::r-x
	default:user::rwx
	default:user:1000:rwx
	default:group::r-x
	default:mask::rwx
	default:other::r-x

	A::OWNER@:rwaDxtTcCy
	A::1000:rwaDxtcy
	A::GROUP@:rxtcy
	A::EVERYONE@:rxtcy
	A:fdi:OWNER@:rwaDxtTcCy
	A:fdi:1000:rwaDxtcy
	A:fdi:GROUP@:rxtcy
	A:fdi:EVERYONE@:rxtcy

Reported-by: Ondrej Valousek <ondrej.valousek@diasemi.com>
Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2136452
Suggested-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>