s390x: fixing I/O memory allocation

The allocator allocates pages, so the size must be rounded up
to whole pages before the allocation.

Fixes: b0fe3988 "s390x: define UV compatible I/O allocation"

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Message-Id: <20211111100153.86088-1-pmorel@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>