qnx4fs: Use kmemdup rather than duplicating its implementation

The semantic patch that makes this change is available
in scripts/coccinelle/api/memdup.cocci.

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Anders Larsen <al@alarsen.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>