tools lib traceevent: Fix get_field_str() for dynamic strings

tools lib traceevent: Fix get_field_str() for dynamic strings

BugLink: http://bugs.launchpad.net/bugs/1774063
[ Upstream commit d777f8de99b05d399c0e4e51cdce016f26bd971b ]

If a field is a dynamic string, get_field_str() returned just the
offset/size value and not the string. Have it parse the offset/size
correctly to return the actual string. Otherwise filtering fails when
trying to filter fields that are dynamic strings.

Reported-by: Gopanapalli Pradeep <prap_hai@yahoo.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20180112004823.146333275@goodmis.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>