f2fs: introduce inmem curseg

Previous implementation of aligned pinfile allocation will:
- allocate new segment on cold data log no matter whether last used
segment is partially used or not, it makes IOs more random;
- force concurrent cold data/GCed IO going into warm data area, it
can make a bad effect on hot/cold data separation;

In this patch, we introduce a new type of log named 'inmem curseg',
the differents from normal curseg is:
- it reuses existed segment type (CURSEG_XXX_NODE/DATA);
- it only exists in memory, its segno, blkofs, summary will not b
 persisted into checkpoint area;

With this new feature, we can enhance scalability of log, special
allocators can be created for purposes:
- pure lfs allocator for aligned pinfile allocation or file
defragmentation
- pure ssr allocator for later feature

So that, let's update aligned pinfile allocation to use this new
inmem curseg fwk.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -1656,13 +1656,14 @@ next_alloc:
 		}
 
 		down_write(&sbi->pin_sem);
-		map.m_seg_type = CURSEG_COLD_DATA_PINNED;
 
 		f2fs_lock_op(sbi);
-		f2fs_allocate_new_segment(sbi, CURSEG_COLD_DATA);
+		f2fs_allocate_new_segment(sbi, CURSEG_COLD_DATA_PINNED);
 		f2fs_unlock_op(sbi);
 
+		map.m_seg_type = CURSEG_COLD_DATA_PINNED;
 		err = f2fs_map_blocks(inode, &map, 1, F2FS_GET_BLOCK_PRE_DIO);
+
 		up_write(&sbi->pin_sem);
 
 		done += map.m_len;