ANDROID: Incremental fs: Optimize get_filled_block

This led to a 20x speed improvement on QEMU. 512 is somewhat
arbitrary - most of the gains are already there reading 64 records
at a time, but since the record size is 10 bytes, 512 is just over
a page and seems a good choice.

Bug: 153170997
Test: incfs_test passes. Adding logging to incfs_get_filled_blocks
	to measure performance shows a 20x improvement
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: Ifb2da77cfd8c9d653c7047ba1eb7f39d795fa1c2