Use kstrdup rather than duplicating its implementation

The semantic patch that makes this output is available
in scripts/coccinelle/api/kstrdup.cocci.

More information about semantic patching is available at
http://coccinelle.lip6.fr/Signed-off-by: NThomas Meyer <thomas@m3y3r.de>
Signed-off-by: NTony Lindgren <tony@atomide.com>