sparc32: do not include swap.h from pgtable_32.h

"memcg: export struct mem_cgroup" will add includes into
linux/memcontrol.h which lead to further header dependency issues as
reported by Guenter Roeck:

  In file included from include/linux/highmem.h:7:0,
                   from include/linux/bio.h:23,
                   from include/linux/writeback.h:192,
                   from include/linux/memcontrol.h:30,
                   from include/linux/swap.h:8,
                   from ./arch/sparc/include/asm/pgtable_32.h:17,
                   from ./arch/sparc/include/asm/pgtable.h:6,
                   from arch/sparc/kernel/traps_32.c:23:
  include/linux/mm.h: In function 'is_vmalloc_addr':
  include/linux/mm.h:371:17: error: 'VMALLOC_START' undeclared (first use in this function)
  include/linux/mm.h:371:17: note: each undeclared identifier is reported only once for each function it appears in
  include/linux/mm.h:371:41: error: 'VMALLOC_END' undeclared (first use in this function)
  include/linux/mm.h: In function 'maybe_mkwrite':
  include/linux/mm.h:556:3: error: implicit declaration of function 'pte_mkwrite'

The issue is that pgtable_32.h depends on swap.h to get swap_entry_t but
that goes all the way down to linux/mm.h which wants to have VMALLOC_*
which is defined later in pgtable_32.h, though.

swap_entry_t is defined in include/mm_types.h so it should be sufficient
to include this header without more dependencies.

Signed-off-by: Michal Hocko <mhocko@suse.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>