Fix missing mark_page_accessed() for metadata

While testing by

	dd if=/mnt/file of=/dev/null bs=4K

with blktrace, we found tux3 is re-reading 2 metadata blocks.

To fix re-read of metadata, this updates LRU info of accessed metadata
via blockget()/blockread().

With this patch, there is no measurable performance difference
though. This removed re-reading 2 metadata blocks from blktrace.

FIXME: mark_page_accessed() for each blockget()/blockread() would be
overkill. Because data pages calls mark_page_accessed() for each page,
not buffer.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>