[S390] vmur: reject open on z/VM reader files with status HOLD

[S390] vmur: reject open on z/VM reader files with status HOLD

If a reader file with HOLD status is at the top of the reader queue, currently
all read requests will return data of the second file in the queue. But the
semantics of vmur is that always the topmost file is read. With this fix
-EPERM is returned on open, if the topmost reader file is in HOLD status.

Signed-off-by: Michael Holzheu <holzheu@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>