s390/hypfs: add interface for diagnose 0x304

To provide access to the set-partition-resource-parameter interface
to user space add a new attribute to hypfs/debugfs:
 * s390_hypsfs/diag_304
The data for the query-partition-resource-parameters command can
be access by a read on the attribute. All other diagnose 0x304
requests need to be submitted via ioctl with CAP_SYS_ADMIN rights.

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