When there is a need to modify only timestamps, it is much simpler and
quicker to do it to the existing file rather than re-write all the
contents.

In preparation for that, add the ability to modify the input file in place.
In practice that just means making the file descriptor and mmaps writable.
Signed-off-by: NAdrian Hunter <adrian.hunter@intel.com>
Reviewed-by: NAndi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: https://lore.kernel.org/r/20210430070309.17624-5-adrian.hunter@intel.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>