f2fs: compress: fix potential deadlock

f2fs: compress: fix potential deadlock

generic/269 reports a hangtask issue, the root cause is ABBA deadlock
described as below:

Thread A			Thread B
- down_write(&sbi->gc_lock) -- A
				- f2fs_write_data_pages
				 - lock all pages in cluster -- B
				 - f2fs_write_multi_pages
				  - f2fs_write_raw_pages
				   - f2fs_write_single_data_page
				    - f2fs_balance_fs
				     - down_write(&sbi->gc_lock) -- A
- f2fs_gc
 - do_garbage_collect
  - ra_data_block
   - pagecache_get_page -- B

To fix this, it needs to avoid calling f2fs_balance_fs() if there is
still cluster pages been locked in context of cluster writeback, so
instead, let's call f2fs_balance_fs() in the end of
f2fs_write_raw_pages() when all cluster pages were unlocked.

Fixes: 4c8ff7095bef ("f2fs: support data compression")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>