nilfs2: segment usage file

This adds a meta data file which stores the allocation state of segments.

[konishi.ryusuke@lab.ntt.co.jp: fix wrong counting of checkpoints and dirty segments]
Signed-off-by: Koji Sato <sato.koji@lab.ntt.co.jp>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nilfs2/cpfile.c

@@ -357,7 +357,7 @@ int nilfs_cpfile_delete_checkpoints(struct inode *cpfile,
 		kaddr = kmap_atomic(header_bh->b_page, KM_USER0);
 		header = nilfs_cpfile_block_get_header(cpfile, header_bh,
 						       kaddr);
-		le64_add_cpu(&header->ch_ncheckpoints, -tnicps);
+		le64_add_cpu(&header->ch_ncheckpoints, -(u64)tnicps);
 		nilfs_mdt_mark_buffer_dirty(header_bh);
 		nilfs_mdt_mark_dirty(cpfile);
 		kunmap_atomic(kaddr, KM_USER0);