powerpc/uaccess: Cast away __user annotation after verification

Sparse reports dereference of a __user pointer. copy_mc_to_user() takes
a __user pointer, verifies it, then calls the generic copy routine
copy_mc_generic().

As we have verified the pointer, cast out the __user annotation when
passing to copy_mc_generic().

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231011053711.93427-10-bgray@linux.ibm.com