Adjust the size computation for reserved blocks

Due to the change in https://lwn.net/Articles/546473/, kernel reserves a
few extra blocks (lesser of 2% and 4096 blocks) on ext4 FS which leads to
OTA update failures. Adjust the size computation if the device has
BOARD_HAS_EXT4_RESERVED_BLOCKS := true.

Bug: 21522719
Bug: 22023465
Change-Id: I49f16adbf2dedc5279fbb8622bf99ef71dcc494f