in sys_read(), log_write() may be called without begin_op()

[jeehoonkang]

I think, in sys_read(), log_write() may be called without begin_op(), which may panic due to an assertion: https://github.com/mit-pdos/xv6-riscv/blob/riscv/kernel/log.c#L223

The function call chain I suspect is: sys_read -> fileread -> readi -> bmap -> balloc -> log_write .
Even though a file is only read, it may need to zero-initialize new blocks.