netfilter: nf_tables: eliminate useless condition checks

The return value of nf_tables_table_lookup() is valid pointer or one
pointer error. There are two cases:

1) IS_ERR(table) is true, it would return the error or reset the
   table as NULL, it is unnecessary to perform the latter check
   "table != NULL".

2) IS_ERR(obj) is false, the table is one valid pointer. It is also
   unnecessary to perform that check.

The nf_tables_newset() and nf_tables_newobj() have same logic codes.

In summary, we could move the block of condition check "table != NULL"
in the else block to eliminate the original condition checks.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>