perf top browser: Adjust the browser indexes when refreshing

This is not a problem when we're not at the bottom of the active symbols
list, so was not noticed, but at the end of the screen it falls apart.

Fix it by adjusting the ui_browser indexes according to the new number
of entries in the rb_tree and by seeking from the start of the rb_tree
to find the new symbol at the top of the screen.

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: 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>