f2fs: fix potential deadlock in gc_merge path of f2fs_balance_fs()

f2fs: fix potential deadlock in gc_merge path of f2fs_balance_fs()

When we mount device w/ gc_merge mount option, we may suffer below
potential deadlock:

Kworker					GC trehad			Truncator
- f2fs_write_cache_pages
 - f2fs_write_single_data_page
  - f2fs_do_write_data_page
   - folio_start_writeback  --- set writeback flag on folio
   - f2fs_outplace_write_data
   : cached folio in internal bio cache
  - f2fs_balance_fs
   - wake_up(gc_thread)
   : wake up gc thread to run foreground GC
   - finish_wait(fggc_wq)
   : wait on the waitqueue --- wait on GC thread to finish the work
									- truncate_inode_pages_range
									 - __filemap_get_folio(, FGP_LOCK)  --- lock folio
									 - truncate_inode_partial_folio
									  - folio_wait_writeback            --- wait on writeback being cleared
					- do_garbage_collect
					 - move_data_page
					  - f2fs_get_lock_data_folio
					   - lock on folio  --- blocked on folio's lock

In order to avoid such deadlock, let's call below functions to commit
cached bios in GC_MERGE path of f2fs_balance_fs() as the same as we did
in NOGC_MERGE path.
- f2fs_submit_merged_write(sbi, DATA);
- f2fs_submit_all_merged_ipu_writes(sbi);

Cc: stable@kernel.org
Fixes: 351df4b20115 ("f2fs: add segment operations")
Cc: Ruipeng Qi <ruipengqi3@gmail.com>
Reported: Sandeep Dhavale <dhavale@google.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Chao Yu <chaseyu@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>