mkdir_p: account for '//foo/bar'

As Richard reported, dirname('//') returns //.  But mkdir_p only stops
when called with '/', resulting in infinite recursion when given a
pathname '//foo/bar'.

Reported-by: richard -rw- weinberger <richard.weinberger@gmail.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
fix spacing

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>