perf evlist: Clarify sterror_mmap variable names

perf evlist: Clarify sterror_mmap variable names

Prep patch for doing further checks like when the number of pages that
is being attempted is actually below /proc/sys/kernel/perf_event_mlock_kb but
the operation fails because the user doesn't have CAP_IPC_LOCK.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-wetzlux7mzvofu5cuji5i71i@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>