Upload failure in posix backend when filesystem is cifs.

[rhamon]

The bug Cannot upload files to versitygw when using the posix backend on cifs mounts.

To Reproduce Setup:

$ versitygw posix /mnt/cifs_mount
No IAM service configured, enabling single account mode
 ┌───────────────────────────────────────────────────┐
 │                     versitygw                     │
 │                   Fiber v2.51.0                   │
 │               http://127.0.0.1:7070               │
 │       (bound on host 0.0.0.0 and port 7070)       │
 │                                                   │
 │ Handlers ............ 25  Processes ........... 1 │
 │ Prefork ....... Disabled  PID ............. 12345 │
 └───────────────────────────────────────────────────┘

Client:

$ aws --endpoint=http://127.0.0.1:7070 s3 mb s3://bucket
make_bucket: bucket
$ aws --endpoint=http://127.0.0.1:7070 s3 cp testfile s3://bucket/testfile
upload failed: ./upload.txt to s3://signage/upload.txt An error occurred (ExistingObjectIsDirectory) when calling the PutObject operation: Existing Object is a directory.
$ ls /mnt/cifs_mount/bucket/
drwxr-xr-x 2 root root 0 Jan  1 00:00 .
drwxr-xr-x 2 root root 0 Jan  1 00:00 ..
drwxr-xr-x 2 root root 0 Jan  1 00:00 .sgwtmp

Server output:

00:00:00 | 200 |   19.209157ms |       127.0.0.1 | PUT     | /bucket
00:00:01 | 409 |  111.251961ms |       127.0.0.1 | PUT     | /bucket/testfile

Expected behavior Client:

$ aws --endpoint=http://127.0.0.1:7070 s3 cp testfile s3://bucket/testfile
upload: ./testfile to s3://bucket/testfile
$ ls /mnt/cifs_mount/bucket/
drwxr-xr-x 3 root root 4.0K Jan  5 00:00 .
drwxr-xr-x 3 root root 4.0K Jan  2 00:00 ..
drwx------ 2 root root 4.0K Jan  2 00:00 .sgwtmp
-rw------- 1 root root    8 Feb  2 00:00 testfile

Server output:

00:00:00 | 200 |   19.209157ms |       127.0.0.1 | PUT     | /bucket
00:00:01 | 200 |  111.251961ms |       127.0.0.1 | PUT     | /bucket/testfile

Server Version Version : 0.11 Build : 8b31d6d93c76ef5d2d563047518c1e91b28d38c0 BuildTime: 2023-12-30T05:41:01Z

Additional context Debian 11.7 cifs-utils 2:6.11-3.1+deb11u1 awscli 1.19.1-1 CIFS share from Windows Server 2016