Fix incorrect checksums for small initial writes in the old store

Details:
- Write size should be exactly csum_block_size (4k by default)
- Write should be made into a new object (unallocated space)
- In this case, the checksum of the block was calculated as if the block was
  padded with extra (2^32 - size) zero bytes due to a simple integer overflow
- As a cherry on the cake, such calculation was 'slightly' slow because it
  was processing almost 4 GB of zeroes for a small write

src/blockstore/v1/write.cpp

@@ -183,7 +183,7 @@ bool blockstore_impl_t::enqueue_write(blockstore_op_t *op)
         uint32_t end = (op->offset+op->len-1) / dsk.csum_block_size;
         auto fn = state & BS_ST_BIG_WRITE ? crc32c_pad : crc32c_nopad;
         if (start == end)
-            data_csums[0] = fn(0, op->buf, op->len, op->offset - start*dsk.csum_block_size, end*dsk.csum_block_size - (op->offset+op->len));
+            data_csums[0] = fn(0, op->buf, op->len, op->offset - start*dsk.csum_block_size, (end+1)*dsk.csum_block_size - (op->offset+op->len));
         else
         {
             // First block

src/util/crc32c.c

@@ -37,6 +37,7 @@
 #include <stdlib.h>
 #include <stdint.h>
 #include <unistd.h>
+#include <assert.h>
 #include "crc32c.h"
 
 #ifdef WITH_ISAL
@@ -394,6 +395,7 @@ static uint8_t zero_page[4096] = {};
 
 uint32_t crc32c_pad(uint32_t prev_crc, const void *buf, size_t len, size_t left_pad, size_t right_pad)
 {
+    assert(left_pad < 0x10000000 && right_pad < 0x10000000);
     uint32_t r = prev_crc;
     while (left_pad >= 4096)
     {