util: Remove usages of linear_realloc()

Note that for linear allocator, the realloc will always
allocate new memory.  In both cases that realloc was used,
the existing size was known, so we can just allocate
and do the copy ourselves.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25280>

src/util/ralloc.c

@@ -1226,10 +1226,12 @@ linear_vasprintf_rewrite_tail(linear_ctx *ctx, char **str, size_t *start,
 
    new_length = u_printf_length(fmt, args);
 
-   ptr = linear_realloc(ctx, *str, *start + new_length + 1);
+   ptr = linear_alloc_child(ctx, *start + new_length + 1);
    if (unlikely(ptr == NULL))
       return false;
 
+   memcpy(ptr, *str, *start);
+
    vsnprintf(ptr + *start, new_length + 1, fmt, args);
    *str = ptr;
    *start += new_length;
@@ -1245,10 +1247,11 @@ linear_cat(linear_ctx *ctx, char **dest, const char *str, unsigned n)
    assert(dest != NULL && *dest != NULL);
 
    existing_length = strlen(*dest);
-   both = linear_realloc(ctx, *dest, existing_length + n + 1);
+   both = linear_alloc_child(ctx, existing_length + n + 1);
    if (unlikely(both == NULL))
       return false;
 
+   memcpy(both, *dest, existing_length);
    memcpy(both + existing_length, str, n);
    both[existing_length + n] = '\0';