testsuites/samples/fileio/init.c: Address -Wsign-compare warnings

Most of these warnings were between int or ssize_t and size_t.
In particular, various POSIX system calls like read() and write()
return ssize_t and comparing that value with the sizeof a buffer
is a common source of these warnings. Another common source is
using an int as the iterator in a for loop with the limit being
a size_t.

With the type change, some printf() specifiers needed to change also.

testsuites/samples/fileio/init.c

@@ -263,7 +263,7 @@ disk_test_write_blocks (const char *name, int start, int count, size_t size)
   int                 block;
   uint32_t*           ip;
   uint32_t            value = 0;
-  int                 i;
+  size_t              i;
   rtems_bdbuf_buffer* bd;
   rtems_status_code   sc;
   int                 rv = 0;