libkmod: Propagate hash_add errors

libkmod: Propagate hash_add errors

If newly created module in kmod_module_new could not be added to hash,
return an error. Otherwise it is possible to create multiple independent
structs with the same name, which the hash set is supposed to prevent.

An error only occurs in out of memory conditions.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/248
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>