src/core/automount: use DirectoryMode when calling mkdir -p

src/core/automount: use DirectoryMode when calling mkdir -p

mkdir -p is called both when setting up the autofs mount, as well
as after being notified that the real mount unit should be called.
However the first mkdir -p is hardcoded with 0555, while the second
uses the value specified to DirectoryMode in the automount unit; the
second mkdir -p is only needed when called from coldplug, so under
normal operation the dirs are incorrectly created with mode 0555.

This replaces the hardcoded 0555 mode with the value of DirectoryMode.

Closes #13683.