s3 sendfile: Fix two bugs in sendfile

s3 sendfile: Fix two bugs in sendfile

These were found interally via code inspection.

1) fake_sendfile was incorrectly writing zeros over real data on a
   short read.

2) sendfile_short_send was doing 4 byte writes instead of 1024 byte
   writes due to an incorrect sizeof usage.

Jermey, Vl please check