perf script python: Fix buffer size to report iregs in perf script

Commit 48a1f565 ("perf script python: Add more PMU fields to
event handler dict") added functionality to report fields like weight,
iregs, uregs etc via perf report.  That commit predefined buffer size to
512 bytes to print those fields.

But in PowerPC, since we added extended regs support in:

  068aeea3 ("perf powerpc: Support exposing Performance Monitor Counter SPRs as part of extended regs")
  d735599a ("powerpc/perf: Add extended regs support for power10 platform")

Now iregs can carry more bytes of data and this predefined buffer size
can result to data loss in perf script output.

This patch resolves this issue by making the buffer size dynamic, based
on the number of registers needed to print. It also changes the
regs_map() return type from int to void, as it is not being used by the
set_regs_in_dict(), its only caller.

Fixes: 068aeea3

 ("perf powerpc: Support exposing Performance Monitor Counter SPRs as part of extended regs")
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Tested-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Cc: Paul Clarke <pc@us.ibm.com>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lore.kernel.org/lkml/20210628062341.155839-1-kjain@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>