s390/cpum_cf: open access to hwctr device for CAP_PERFMON privileged process

The device /dev/hwctr was introduced to access complete
CPU Measurement facility counter sets via an ioctl system call.
The access the to device is limited to privileged processes
running as root or superuser. The capability CAP_SYS_ADMIN
is required.  The device permissions are read/write for the
device owner root. There is no need for this restriction.

Make the device access permission read/write for all and
reduce the capabilities to CAP_PERFMON.
Any user space program with the CAP_PERFMON capability assigned to it
can now read and display the CPU Measurement facility counter sets.

For more details on perf tool usage and security, see linux
documentation in Documentation/admin-guide/perf-security.rst.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>