[BLKTAP] Various cleanups necessary for ia64 support.

[BLKTAP] Various cleanups necessary for ia64 support.
- remove magic number 8 and 64.
  page size / sector size is 8 on x86, however on ia64 page size is
  16kb.
  blkring size is 64 on x86. however it's 128 on ia64.
- replace 0xFFFF with INVALID_GRANT_HANDLE
- don't map io ring into user space with uncachable.
  kernel access the page with cacable and tapdisk uses memory barrier
  so that it isn't necessary.
- remove printk warning and inserted new line.
- remove unused variable, page.
- add one BUG()

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>