Even though DiskChecker.mkdirsWithExistsCheck() will create the directory tree,
it is only called *after* the enumeration of directories with available
space has completed.
Directories which don't exist are reported as having 0 space, therefore
the mkdirs code is never reached.
Adding a simple mkdirs() -without bothering to check the outcome-
ensures that if a dir has been deleted then it will be reconstructed
if possible. If it can't it will still have 0 bytes of space
reported and so be excluded from the allocation.
Contributed by Steve Loughran
11a220c6e7