lib/btree: simplify btree_{lookup|update}

btree_{lookup|update} both need to look up node by key, using the common
parts(add function btree_lookup_node) to simplify code.

Link: https://lkml.kernel.org/r/20220607133556.34732-1-wuchi.zero@gmail.com


Signed-off-by: wuchi <wuchi.zero@gmail.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>