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>
Fix spell miss of add_defer_oprhan_del()

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Add FIXME as big comment to blockget() for future

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Rename [io]table to [io]tree

We are going to use "tree" for B+tree data structure (at least, in
code), and use "table" for data structure represented in internal file.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Rename from "rollup" to "unify"

We change our terminology from "rollup" to "unify".

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

Now, buffer_index_cmp() vi list_sort() is top cpu user in backend on
some loads.

So, this optimizes buffer_index_cmp().

Open code bufindex(), then remove unnecessary calculate to compare,
and check page->index and buffer->b_data separately.

The result of this optimizations (measured by fsperf),

dd if=/dev/zero of=/mnt/file bs=4K count=1M conv=fsync

Before:

	4294967296 bytes (4.3 GB) copied, 67.4053 s, 63.7 MB/s

	flush-tux3-8:1 used cpu for 1.696215114 secs

After:

	4294967296 bytes (4.3 GB) copied, 67.0487 s, 64.1 MB/s

	flush-tux3-8:1 used cpu for 1.484304951 secs

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