shared: use memdup over stdndupa

shared: use memdup over stdndupa

The stdndupa has a couple of gotchas:
 - allocates memory on stack via alloca(3)... where we pass it a
   user-provided string in at least one instance
 - it's a GNU extension missing on musl and bionic

The mkdir_p() function is not a hot path, so using heap allocation is
perfectly fine. Swap the stdndupa for our local helper memdup.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/92
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>