f2fs: avoid f2fs_bug_on() in cp_error case

There is a subtle race condition to invoke f2fs_bug_on() in shutdown tests. I've
confirmed that the last checkpoint is preserved in consistent state, so it'd be
fine to just return error at this moment.
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -1075,6 +1075,10 @@ int f2fs_remove_inode_page(struct inode *inode)
 		f2fs_truncate_data_blocks_range(&dn, 1);
 
 	/* 0 is possible, after f2fs_new_inode() has failed */
+	if (unlikely(f2fs_cp_error(F2FS_I_SB(inode)))) {
+		f2fs_put_dnode(&dn);
+		return -EIO;
+	}
 	f2fs_bug_on(F2FS_I_SB(inode),
 			inode->i_blocks != 0 && inode->i_blocks != 8);