nilfs2: verify metadata sizes read from disk

Add code to check sizes of on-disk data of metadata files such as
inode size, segment usage size, DAT entry size, and checkpoint size.
Although these sizes are read from disk, the current implementation
doesn't check them.

If these sizes are not sane on disk, it can cause out-of-range access
to metadata or memory access overrun on metadata block buffers due to
overflow in sundry calculations.

Both lower limit and upper limit of metadata sizes are verified to
prevent these issues.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
nilfs2: update MAINTAINERS file entries fix

Also, web-page entry is updated according to relocation of project's
web site.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
nilfs2: update project's web site in nilfs2.txt

Project's web site was moved to nilfs.sourceforge.net from
www.nilfs.org.  This updates the site information in
Documentation/filesystems/nilfs2.txt with the new location.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>