perf inject: Fix itrace space allowed for new attributes

perf inject: Fix itrace space allowed for new attributes

The space allowed for new attributes can be too small if existing header
information is large. That can happen, for example, if there are very
many CPUs, due to having an event ID per CPU per event being stored in the
header information.

Fix by adding the existing header.data_offset. Also increase the extra
space allowed to 8KiB and align to a 4KiB boundary for neatness.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/20211125071457.2066863-1-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>