percpu: fix __percpu annotation in asm-generic

The generic implementation of raw_cpu_generic_add_return() is:

        #define raw_cpu_generic_add_return(pcp, val)            \
        ({                                                      \
                typeof(&(pcp)) __p = raw_cpu_ptr(&(pcp));       \
                                                                \
                *__p += val;                                    \
                *__p;                                           \
        })

where the 'pcp' argument is a __percpu lvalue.
There, the variable '__p' is declared as a __percpu pointer
the type of the address of 'pcp') but:
1) the value assigned to it, the return value of raw_cpu_ptr(), is
   a plain (__kernel) pointer, not a __percpu one.
2) this variable is dereferenced just after while a __percpu
   pointer is implicitly __noderef.

So, fix the declaration of the 'pcp' variable to its correct type:
the plain (non-percpu) pointer corresponding to pcp's address,
using the...