Sometimes, after running generic/270 of fstest, fsck reports summary
info and actual position of block address in direct node becoming
inconsistent.

The root cause is race in between __f2fs_replace_block and change_curseg
as below:

Thread A				Thread B
- __clone_blkaddrs
 - f2fs_replace_block
  - __f2fs_replace_block
   - segnoA = GET_SEGNO(sbi, blkaddrA);
   - type = se->type:=CURSEG_HOT_DATA
   - if (!IS_CURSEG(sbi, segnoA))
         type = CURSEG_WARM_DATA
					- allocate_data_block
					 - allocate_segment
					  - get_ssr_segment
					  - change_curseg(segnoA, CURSEG_HOT_DATA)
   - change_curseg(segnoA, CURSEG_WARM_DATA)
    - reset_curseg
     - __set_sit_entry_type
      - change se->type from CURSEG_HOT_DATA to CURSEG_WARM_DATA

So finally, hot curseg locates in segnoA, but type of segnoA becomes
CURSEG_WARM_DATA.

Then if we invoke __f2fs_replace_block(blkaddrB, blkaddrA, true, false),
as blkaddrA locates in segnoA, so we will move warm type curseg to segnoA,
then change its summary cache and writeback it to summary block.

But segnoA is used by hot type curseg too, once it moves or persist, it
will cover summary block content with inner old summary cache, result in
inconsistent status.

This patch tries to fix this issue by introduce global curseg lock to avoid
race in between __f2fs_replace_block and change_curseg.

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