nfs: Fix NFS v4 client handling of MAY_EXEC in nfs_permission.
The problem is that permission checking is skipped if atomic open is
possible, but when exec opens a file, it just opens it O_READONLY which
means EXEC permission will not be checked at that time.
This problem is observed by the following sequence (executed as root):
mount -t nfs4 server:/ /mnt4
echo "ls" >/mnt4/foo
chmod 744 /mnt4/foo
su guest -c "mnt4/foo"