Fix BUG#11798085 - INCORRECT INTEGER TYPES USED IN CALCULATION RESULT

IN OVERFLOW

Do not assign the result of the difference to a signed variable and
checking whether it is negative afterwards because this limits the max diff
to 2G on 32 bit systems. E.g. "signed = 3.5G - 1G" would be negative and the
code would assume that 3.5G < 1G. Instead compare the two variables directly
and assign to unsigned only if we know that the result of the subtraction
will be positive.

Discussed with:	Jimmy and Sunny (via IRC)

storage/innodb_plugin/buf/buf0buf.c

@@ -1893,16 +1893,19 @@ buf_block_align(
 	/* TODO: protect buf_pool->chunks with a mutex (it will
 	currently remain constant after buf_pool_init()) */
 	for (chunk = buf_pool->chunks, i = buf_pool->n_chunks; i--; chunk++) {
-		lint	offs = ptr - chunk->blocks->frame;
+		ulint	offs;
 
-		if (UNIV_UNLIKELY(offs < 0)) {
+		if (UNIV_UNLIKELY(ptr < chunk->blocks->frame)) {
 
 			continue;
 		}
+		/* else */
+
+		offs = ptr - chunk->blocks->frame;
 
 		offs >>= UNIV_PAGE_SIZE_SHIFT;
 
-		if (UNIV_LIKELY((ulint) offs < chunk->size)) {
+		if (UNIV_LIKELY(offs < chunk->size)) {
 			buf_block_t*	block = &chunk->blocks[offs];
 
 			/* The function buf_chunk_init() invokes