client: don't send uninitialized data in fio_send_file error replies

Various error cases in fio_send_file() might send an uninitialized
buffer over the wire. The server should ignore this when errno is set,
but it's best practice to not ship heap data over the wire, so fix that
by:

- using calloc vs malloc in the initial allocation so that early error
  paths send initialized data.

- grow the buffer via realloc only after open() succeeds, so that an
  open() failure doesn't send a grown/uninit buffer back with the error.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Link: https://patch.msgid.link/20260917205554.2281093-1-sandeen@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>