f2fs: fix to wait page writeback before setting gcing flag

Soft IRQ				Thread
- f2fs_write_end_io
					- f2fs_defragment_range
					 - set_page_private_gcing
 - type = WB_DATA_TYPE(page, false);
 : assign type w/ F2FS_WB_CP_DATA
 due to page_private_gcing() is true
  - dec_page_count() w/ wrong type
  - end_page_writeback()

Value of F2FS_WB_CP_DATA reference count may become negative under above
race condition, the root cause is we missed to wait page writeback before
setting gcing page private flag, let's fix it.

Fixes: 2d1fe8a86b ("f2fs: fix to tag gcing flag on page during file defragment")
Fixes: 4961acdd65 ("f2fs: fix to tag gcing flag on page during block migration")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -2807,6 +2807,8 @@ do_map:
 				goto clear_out;
 			}
 
+			f2fs_wait_on_page_writeback(page, DATA, true, true);
+
 			set_page_dirty(page);
 			set_page_private_gcing(page);
 			f2fs_put_page(page, 1);
@@ -4207,6 +4209,8 @@ static int redirty_blocks(struct inode *inode, pgoff_t page_idx, int len)
 		/* It will never fail, when page has pinned above */
 		f2fs_bug_on(F2FS_I_SB(inode), !page);
 
+		f2fs_wait_on_page_writeback(page, DATA, true, true);
+
 		set_page_dirty(page);
 		set_page_private_gcing(page);
 		f2fs_put_page(page, 1);