Avoid ListBuckets to keep Stat() ops on bucket simpler

[harshavardhana]

Due to multi-user feature on Minio and mc adoption in
many different restrictive environments, most users disable
access to ListBuckets() calls. We can avoid such network
operations as they only add marginal value.

[codecov]

Codecov Report

Merging #2699 into master will decrease coverage by 0.02%.
The diff coverage is 14.7%.

@@            Coverage Diff            @@
##           master   #2699      +/-   ##
=========================================
- Coverage    9.69%   9.66%   -0.03%     
=========================================
  Files         128     128              
  Lines       12109   12093      -16     
=========================================
- Hits         1174    1169       -5     
+ Misses      10785   10772      -13     
- Partials      150     152       +2

Impacted Files	Coverage Δ
cmd/client-s3.go	13.07% <14.7%> (-0.21%)	⬇️

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[codecov]

Codecov Report

Merging #2699 into master will decrease coverage by 0.14%.
The diff coverage is 14.7%.

@@            Coverage Diff            @@
##           master   #2699      +/-   ##
=========================================
- Coverage    9.69%   9.54%   -0.15%     
=========================================
  Files         128     126       -2     
  Lines       12109   12055      -54     
=========================================
- Hits         1174    1151      -23     
+ Misses      10785   10760      -25     
+ Partials      150     144       -6

Impacted Files	Coverage Δ
cmd/client-s3.go	13.07% <14.7%> (-0.21%)	⬇️
cmd/client-fs.go	27.73% <0%> (-1.74%)	⬇️
cmd/client-url.go	66.66% <0%> (-1.45%)	⬇️
cmd/config.go	34.06% <0%> (-0.55%)	⬇️
cmd/fs-pathutils_nix.go
cmd/client-fs_linux.go

---

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[poornas]

Tested with a getonly policy for multi-user, without ListBuckets access, works well. LGTM

[kannappanr]

Tested with the following policy that works on a bucket named bucket

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:GetBucketLocation",
        "s3:GetObject",
        "s3:ListBucket"
      ],
      "Resource": [
        "arn:aws:s3:::bucket/*"
      ]
    }
  ]
}

Failed with buckets not named bucket as expected.