oom_get_memory reads the whole page, for every page of dynamic data - optimize!

[donboll]

oom_get_memory(port, 'DOM') needs to read 10 bytes (SFP) or 36 bytes (QSFP), but it always reads 128 bytes to fill the page cache. Try to optimize this. It should be at least twice as fast on QSFP, four times faster on SFP.