[PATCH] blktrace: Don't overwrite buf_size for the vbuffer

[PATCH] blktrace: Don't overwrite buf_size for the vbuffer

Here's another patch to fix a problem I noticed - the code that sets
the buffer size passed to setvbuf() uses the global buf_size variable
and thus unintentionally sets that variable, which is used elsewhere
later e.g. setting tip->fd_max_size in extract().