Reduce logging around segfaults

The excessive logging of pages with segfaults is making it hard to
see what caused the problem, only log the first page that segfaults,
the range that was being walked when the first segfault happened,
and the total number of pages that segfaulted.

Bug: 120032857
Test: memunreachable_test --gtest_filter=HeapWalkerTest.segv
Change-Id: I71821a3f5be65f2fbcb36afc4b7b1ffa4a48e660