fuse: llseek fix race

Fix race between lseek(fd, 0, SEEK_CUR) and read/write.  This was fixed in
generic code by commit 5b6f1eb9 (vfs: lseek(fd, 0, SEEK_CUR) race condition).
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>

fs/fuse/file.c

@@ -1567,6 +1567,10 @@ static loff_t fuse_file_llseek(struct file *file, loff_t offset, int origin)
 		offset += i_size_read(inode);
 		break;
 	case SEEK_CUR:
+		if (offset == 0) {
+			retval = file->f_pos;
+			goto exit;
+		}
 		offset += file->f_pos;
 		break;
 	case SEEK_DATA: