MIPS: pgtable: fix -Wshadow in asm/pgtable.h

Solves the following repetitive warning when building with -Wshadow:

In file included from ./include/linux/pgtable.h:6,
                 from ./include/linux/mm.h:33,
                 from ./include/linux/dax.h:6,
                 from ./include/linux/mempolicy.h:11,
                 from kernel/fork.c:34:
./arch/mips/include/asm/mmu_context.h: In function ‘switch_mm’:
./arch/mips/include/asm/pgtable.h:97:16: warning: declaration of ‘flags’ shadows a previous local [-Wshadow]
   97 |  unsigned long flags;      \
      |                ^~~~~
./arch/mips/include/asm/mmu_context.h:162:2: note: in expansion of macro ‘htw_stop’
  162 |  htw_stop();
      |  ^~~~~~~~
In file included from kernel/fork.c:102:
./arch/mips/include/asm/mmu_context.h:159:16: note: shadowed declaration is here
  159 |  unsigned long flags;
      |                ^~~~~

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>