f2fs: fix livelock calling f2fs_iget during f2fs_evict_inode

In f2fs_evict_inode,
 commit_inmemory_pages
   f2fs_gc
     f2fs_iget
       iget_locked
         -> wait for inode free

Here, if the inode is same as the one to be evicted, f2fs should wait forever.
Actually, we should not call f2fs_balance_fs during f2fs_evict_inode to avoid
this.

But, the commit_inmem_pages calls f2fs_balance_fs by default, even if
f2fs_evict_inode wants to free inmemory pages only.

Hence, this patch adds to trigger f2fs_balance_fs only when there is something
to write.
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -230,7 +230,16 @@ void commit_inmem_pages(struct inode *inode, bool abort)
 		.rw = WRITE_SYNC,
 	};
 
-	f2fs_balance_fs(sbi);
+	/*
+	 * The abort is true only when f2fs_evict_inode is called.
+	 * Basically, the f2fs_evict_inode doesn't produce any data writes, so
+	 * that we don't need to call f2fs_balance_fs.
+	 * Otherwise, f2fs_gc in f2fs_balance_fs can wait forever until this
+	 * inode becomes free by iget_locked in f2fs_iget.
+	 */
+	if (!abort)
+		f2fs_balance_fs(sbi);
+
 	f2fs_lock_op(sbi);
 
 	mutex_lock(&fi->inmem_lock);