vfs_vnops.c: Fix the named attribute check for open

In vn_open_cred(), the correct check for O_NAMEDATTR
was done when O_CREAT was specified, but the file already
exists. (Added by commit 2ec2ba7e232d, which will be listed
as a Fixes: in the commit log message.)

This correct check was not copied to the case where O_CREAT
has not been specified.

This patch fixes this.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D49898
Fixes: 2ec2ba7e232d ("vfs: Add VFS/syscall support for Solaris style extended attributes")