logger.h: reduce maximum payload so sum is page size

Reduce maximum log payload so that the sum with the maximum
length header logger_entry_v4 is a page size. This is a
performance measure.

Expect side effects, truncates messages 8 bytes earlier. The
length has a long history (2008) so there may be non-C code
somewhere that has it hard-coded rather than reflecting this
header. CTS tests may need to be adjusted.

Bug: 25996918
Change-Id: I598c8c5cc78f6b0afc2c1f30cc6f3c18eb8c4c9d