x86/resctrl: Calculate bandwidth from the previous __mon_event_count() chunks

mbm_bw_count() is only called by the mbm_handle_overflow() worker once a
second. It reads the hardware register, calculates the bandwidth and
updates m->prev_bw_msr which is used to hold the previous hardware register
value.

Operating directly on hardware register values makes it difficult to make
this code architecture independent, so that it can be moved to /fs/,
making the mba_sc feature something resctrl supports with no additional
support from the architecture.
Prior to calling mbm_bw_count(), mbm_update() reads from the same hardware
register using __mon_event_count().

Change mbm_bw_count() to use the current chunks value most recently saved
by __mon_event_count(). This removes an extra call to __rmid_read().
Instead of using m->prev_msr to calculate the number of chunks seen,
use the rr->val that was updated by __mon_event_count(). This removes an
extra call to mbm_overflow_count() and get_corrected_mbm_count().
Calculating bandwidth like this means mbm_bw_count() no longer operates
on hardware register values directly.

Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jamie Iles <quic_jiles@quicinc.com>
Reviewed-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Xin Hao <xhao@linux.alibaba.com>
Tested-by: Shaopeng Tan <tan.shaopeng@fujitsu.com>
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20220902154829.30399-13-james.morse@arm.com