s390/vmlogrdr: change return value from -ENOSYS to -EOPNOTSUPP

Changing the return value of vmlogrdr_open() to -EOPNOTSUPP if O_NONBLOCK
is specified shouldn't have any negative side effects.
Any existing user wouldn't specify that flag since it wouldn't work anyway.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>