Input: uinput - use memdup_user() and friends

Instead of open-coding copying of data structures from userspace use
memdup_user() and strndup_user(). Note that this introduces change in
behavior because driver used to truncate 'phys' longer than 1024 bytes,
but now it will refuse to set 'phys' that long. Arguably trying to set
such 'phys' is suspect anyways.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>