<linux/uaccess.h>: Fix copy_in_user() declaration

copy_in_user() copies data from user-space address @from to user-
space address @to. Hence declare both @from and @to as user-space
pointers.

Fixes: commit d597580d ("generic ...copy_..._user primitives")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>