Cosmetic changes to linker allocator and config

Make the num_free_blocks calculation clearer. There are
sizeof(page->bytes) bytes available for blocks, which is equal to
(kAllocateSize - 16). The previous calculation overestimates the amount
of space by 8 or 12 bytes, but that doesn't change the result because
both kAllocateSize and block_size_ are multiples of 16.

Bug: none
Test: boot device, bionic unit tests
Change-Id: Ia880cd5abc2a7bb4e9e6a8c0441d985f4df8fdc4
diff --git a/linker/linker_config.cpp b/linker/linker_config.cpp
index 5a728d3..ba48b51 100644
--- a/linker/linker_config.cpp
+++ b/linker/linker_config.cpp
@@ -80,7 +80,7 @@
         continue;
       }
 
-      if (line[0] == '[' && line[line.size() - 1] == ']') {
+      if (line[0] == '[' && line.back() == ']') {
         *name = line.substr(1, line.size() - 2);
         return kSection;
       }