vfs_vnops.c: Modify the O_NAMEDATTR check for Solaris compatibility

The Solaris semantics for their O_XATTR flag is to use it
for a file object in the file system's namespace to indicate
that a named attribute for the file object should be open'd.

To do this, the O_NAMEDATTR flag must be allowed with a
non-named attribute directory. This patch changes vfs_vnops_cred()
to allow this.

This patch fixes 2ec2ba7e232d so that Solaris compatible
semantics can be implemented by patched ZFS code.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D49899
Fixes: 2ec2ba7e232d ("vfs: Add VFS/syscall support for Solaris style extended attributes")