perf inject: Refactor read_buildid function

perf inject: Refactor read_buildid function

Into two functions, one that actually reads the build_id for the dso if
it wasn't already read, and another taht will inject the event if the
build_id is available.

Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>