perf events: Account PERF_RECORD_LOST events in event__process

Right now this function is only used by perf top, that uses PROT_READ
only, i.e. overwrite mode, so no PERF_RECORD_LOST events are generated,
but don't forget those events.

The patch that moved this out of perf top was made so that this routine
could be used by 'perf probe' in the uprobes patchset, so perhaps there
they need to check for LOST events and warn the user, as will be done in
the following patches that will switch 'perf top' to non overwrite mode
(mmap with PROT_READ|PROT_WRITE).

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>