Incorrect multipart threshold

[jasoncwik]

Currently, if PutObject is executed with exactly partSize / minPartSize, it will execute a multipart upload with exactly one part. This is wasteful and causes performance issues, especially over the WAN, due to multiple round-trip operations.

Example:

1. Create object exactly 16MiB

/tmp/minio-examples/examples/s3 $ dd if=/dev/random of=my-testfile bs=1M count=16
16+0 records in
16+0 records out
16777216 bytes (17 MB, 16 MiB) copied, 0.0712859 s, 235 MB/s

2. Upload with sample PutObject call

/tmp/minio-examples/examples/s3 $ ls -l my-testfile 
-rw-rw-r-- 1 jcwik jcwik 16777216 Aug 28 09:27 my-testfile

/tmp/minio-examples/examples/s3 $ ./putobject | tee put1.txt
<snip>
2024/08/28 10:29:42 Uploaded my-objectname  of size:  16777216 Successfully in  421.786748ms

You can see there are 3 calls made for the upload, creating a MPU with a single part.

/tmp/minio-examples/examples/s3 $ grep -A 1 START-HT put1.txt
---------START-HTTP---------
GET /testbucket/?location= HTTP/1.1
--
---------START-HTTP---------
POST /testbucket/my-objectname?uploads= HTTP/1.1
--
---------START-HTTP---------
PUT /testbucket/my-objectname?partNumber=1&uploadId=ZGE0YzdiNmQtNzNhYS00MzlhLTgyMzEtZThhMjA2MGM3ZThkLmRjNGUwODUzLTcxY2YtNDhlYy1hZTJkLTAxZTYyNDcyNjYyNngxNzI0ODU5MDE5MjcwOTE2NzI3 HTTP/1.1
--
---------START-HTTP---------
POST /testbucket/my-objectname?uploadId=ZGE0YzdiNmQtNzNhYS00MzlhLTgyMzEtZThhMjA2MGM3ZThkLmRjNGUwODUzLTcxY2YtNDhlYy1hZTJkLTAxZTYyNDcyNjYyNngxNzI0ODU5MDE5MjcwOTE2NzI3 HTTP/1.1

I have a PR, will post in a few minutes.