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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.

It amends the last attemp in [1]. Now it computes the used blocks from the
make_ext4fs output, instead of altering its argument.

[1]: commit efbb5d2e.

Bug: 21522719
Bug: 22023465
Bug: 22174684
Change-Id: Iaae6507f6de68a5892f2e3035d330039287b4492
(cherry picked from commit c7a6f1e4)