Currently we have IOPs of these patterns
```
[OS] os.Mkdir play.min.io:9000 /disk1 2.718µs
[OS] os.Mkdir play.min.io:9000 /disk1/data 2.406µs
[OS] os.Mkdir play.min.io:9000 /disk1/data/.minio.sys 4.068µs
[OS] os.Mkdir play.min.io:9000 /disk1/data/.minio.sys/tmp 2.843µs
[OS] os.Mkdir play.min.io:9000 /disk1/data/.minio.sys/tmp/d89c8ceb-f8d1-4cc6-b483-280f87c4719f 20.152µs
```
It can be seen that we can save quite Nx levels such as
if your drive is mounted at `/disk1/minio` you can simply
skip sending an `Mkdir /disk1/` and `Mkdir /disk1/minio`.
Since they are expected to exist already, this PR adds a way
for us to ignore all paths upto the mount or a directory which
ever has been provided to MinIO setup.
a/b/c/d/ where `a/b/c/` exists results in additional syscalls
such as an Lstat() call to verify if the `a/b/c/` exists
and its a directory.
We do not need to do this on MinIO since the parent prefixes
if exist, we can simply return success without spending
additional syscalls.
Also this implementation attempts to simply use Access() calls
to avoid os.Stat() calls since the latter does memory allocation
for things we do not need to use.
Access() is simpler since we have a predictable structure on
the backend and we know exactly how our path structures are.
Klaus Post
Harshavardhana
jiuker