x86/mm/pat: Clean up <asm/memtype.h> externs

x86/mm/pat: Clean up <asm/memtype.h> externs

Half of the declarations have an 'extern', half of them not,
use 'extern' consistently.

This makes grepping for APIs easier, such as:

  dagon:~/tip> git grep -E '\<memtype_.*\(' arch/x86/ | grep extern
  arch/x86/include/asm/memtype.h:extern int memtype_reserve(u64 start, u64 end,
  arch/x86/include/asm/memtype.h:extern int memtype_free(u64 start, u64 end);
  arch/x86/include/asm/memtype.h:extern int memtype_kernel_map_sync(u64 base, unsigned long size,
  arch/x86/include/asm/memtype.h:extern int memtype_reserve_io(resource_size_t start, resource_size_t end,
  arch/x86/include/asm/memtype.h:extern void memtype_free_io(resource_size_t start, resource_size_t end);
  arch/x86/mm/pat/memtype.h:extern int memtype_check_insert(struct memtype *entry_new,
  arch/x86/mm/pat/memtype.h:extern struct memtype *memtype_erase(u64 start, u64 end);
  arch/x86/mm/pat/memtype.h:extern struct memtype *memtype_lookup(u64 addr);
  arch/x86/mm/pat/memtype.h:extern int memtype_copy_nth_element(struct memtype *entry_out, loff_t pos);

Signed-off-by: Ingo Molnar <mingo@kernel.org>