Input: joydev - use memdup_user() to duplicate memory from user-space

The memdup_user() helper function can be used to duplicate a memory region
from user-space to kernel-space. There is no need to open code the same
logic using kmalloc() and copy_from_user() instead. This was found with
make coccicheck that reported the following warning:

drivers/input/joydev.c:447:10-17: WARNING opportunity for memdup_user
drivers/input/joydev.c:483:10-17: WARNING opportunity for memdup_user

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>