recovery: fix another misspelling of "file_blocks" as "block_size"

Together with I1efff752a223cc937c3a9e3b6a994545a5696379, this makes it
possible to install small (file_blocks < block_size) updates without
getting mysterious bus errors.

Change-Id: I49df1bcc4615d59bd00678df5765f054ccce6522
Signed-off-by: Michael Livshin <michael@cellrox.com>