Simplify and fix os.MkdirAll() usage

TL;DR: check for IsExist(err) after a failed MkdirAll() is both
redundant and wrong -- so two reasons to remove it.

Quoting MkdirAll documentation:

> MkdirAll creates a directory named path, along with any necessary
> parents, and returns nil, or else returns an error. If path
> is already a directory, MkdirAll does nothing and returns nil.

This means two things:

1. If a directory to be created already exists, no error is
returned.

2. If the error returned is IsExist (EEXIST), it means there exists
a non-directory with the same name as MkdirAll need to use for
directory. Example: we want to MkdirAll("a/b"), but file "a"
(or "a/b") already exists, so MkdirAll fails.

The above is a theory, based on quoted documentation and my UNIX
knowledge.
3. In practice, though, current MkdirAll implementation [1] returns
ENOTDIR in most of cases described in #2, with the exception when
there is a race between MkdirAll and someone else creating the
last component of MkdirAll argument as a file. In this very case
MkdirAll() will indeed return EEXIST.

Because of #1, IsExist check after MkdirAll is not needed.

Because of #2 and #3, ignoring IsExist error is just plain wrong,
as directory we require is not created. It's cleaner to report
the error now.

[1] https://github.com/golang/go/blob/f9ed2f75/src/os/path.go

Signed-off-by: Kir Kolyshkin <kir@openvz.org>

libnetwork/endpoint.go

@@ -297,12 +297,7 @@ func (ep *endpoint) processOptions(options ...EndpointOption) {
 }
 
 func createBasePath(dir string) error {
-	err := os.MkdirAll(dir, 0644)
-	if err != nil && !os.IsExist(err) {
-		return err
-	}
-
-	return nil
+	return os.MkdirAll(dir, 0644)
 }
 
 func createFile(path string) error {

libnetwork/sandbox/namespace_linux.go

@@ -48,7 +48,7 @@ func init() {
 
 func createBasePath() {
 	err := os.MkdirAll(prefix, 0644)
-	if err != nil && !os.IsExist(err) {
+	if err != nil {
 		panic("Could not create net namespace path directory")
 	}