ANDROID: dm verity fec: fix RS block calculation

A call to do_div was changed in Linux 4.5 to div64_u64 in
verity_fec_decode, which broke RS block calculation due to
incompatible semantics. This change fixes the computation.

Bug: 21893453
Change-Id: Idb88b901e0209c2cccc9c0796689f780592d58f9
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
(cherry picked from commit 879aac93eebcc2862d71afa9eca3a0c0f51b3b01)