netfilter: nf_tables: Fix percpu address space issues in nf_tables_api.c

Compiling nf_tables_api.c results in several sparse warnings:

nf_tables_api.c:2077:31: warning: incorrect type in return expression (different address spaces)
nf_tables_api.c:2080:31: warning: incorrect type in return expression (different address spaces)
nf_tables_api.c:2084:31: warning: incorrect type in return expression (different address spaces)

nf_tables_api.c:2740:23: warning: incorrect type in assignment (different address spaces)
nf_tables_api.c:2752:38: warning: incorrect type in assignment (different address spaces)
nf_tables_api.c:2798:21: warning: incorrect type in argument 1 (different address spaces)

Use {ERR_PTR,IS_ERR,PTR_ERR}_PCPU() macros when crossing between generic
and percpu address spaces and add __percpu annotation to *stats pointer
to fix these warnings.

Found by GCC's named address space checks.

There were no changes in the resulting object files.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>