arm64: mm: Fix missing-prototypes in pageattr.c

Fix the following warnings.
‘set_memory_valid’ [-Wmissing-prototypes]
int set_memory_valid(unsigned long addr, int numpages, int enable)
     ^
‘set_direct_map_invalid_noflush’ [-Wmissing-prototypes]
int set_direct_map_invalid_noflush(struct page *page)
     ^
‘set_direct_map_default_noflush’ [-Wmissing-prototypes]
int set_direct_map_default_noflush(struct page *page)
     ^

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Link: https://lore.kernel.org/r/1600222847-56792-1-git-send-email-tiantao6@hisilicon.com


arch/arm64/mm/pageattr.c:138:5: warning: no previous prototype for
arch/arm64/mm/pageattr.c:150:5: warning: no previous prototype for
arch/arm64/mm/pageattr.c:165:5: warning: no previous prototype for
Signed-off-by: Will Deacon <will@kernel.org>