perf kmem: Fix compiles on RHEL6/OL6

perf kmem: Fix compiles on RHEL6/OL6

0d68bc92c48 breaks compiles on RHEL6/OL6:
    cc1: warnings being treated as errors
    builtin-kmem.c: In function ‘search_page_alloc_stat’:
    builtin-kmem.c:322: error: declaration of ‘stat’ shadows a global declaration
                            node = &parent->rb_left;
    /usr/include/sys/stat.h:455: error: shadowed declaration is here
    builtin-kmem.c: In function ‘perf_evsel__process_page_alloc_event’:
    builtin-kmem.c:378: error: declaration of ‘stat’ shadows a global declaration
    /usr/include/sys/stat.h:455: error: shadowed declaration is here
    builtin-kmem.c: In function ‘perf_evsel__process_page_free_event’:
    builtin-kmem.c:431: error: declaration of ‘stat’ shadows a global declaration
    /usr/include/sys/stat.h:455: error: shadowed declaration is here

Rename local variable to pstat to avoid the name conflict.

Signed-off-by: David Ahern <david.ahern@oracle.com>
Link: http://lkml.kernel.org/r/1429033773-31383-1-git-send-email-david.ahern@oracle.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>