include/linux/hugetlb.h: convert to use vm_fault_t

kbuild produces the below warning:

  tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
  head:   5453a3df
  commit 3d353901 ("mm: create the new vm_fault_t type")
  reproduce:
        # apt-get install sparse
        git checkout 3d353901
        make ARCH=x86_64 allmodconfig
        make C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__'

  >> mm/memory.c:3968:21: sparse: incorrect type in assignment (different
  >> base types) @@    expected restricted vm_fault_t [usertype] ret @@
  >> got e] ret @@
     mm/memory.c:3968:21:    expected restricted vm_fault_t [usertype] ret
     mm/memory.c:3968:21:    got int

This patch converts to return vm_fault_t type for hugetlb_fault() when
CONFIG_HUGETLB_PAGE=n.

Regarding the sparse warning, Luc said:

: This is the expected behaviour.  The constant 0 is magic regarding bitwise
: types but ({ ...; 0; }) is not, it is just an ordinary expression of type
: 'int'.
:
: So, IMHO, Souptick's patch is the right thing to do.

Link: http://lkml.kernel.org/r/20190318162604.GA31553@jordon-HP-15-Notebook-PCSigned-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/hugetlb.h

@@ -203,7 +203,6 @@ static inline void hugetlb_show_meminfo(void)
 #define pud_huge(x)	0
 #define is_hugepage_only_range(mm, addr, len)	0
 #define hugetlb_free_pgd_range(tlb, addr, end, floor, ceiling) ({BUG(); 0; })
-#define hugetlb_fault(mm, vma, addr, flags)	({ BUG(); 0; })
 #define hugetlb_mcopy_atomic_pte(dst_mm, dst_pte, dst_vma, dst_addr, \
 				src_addr, pagep)	({ BUG(); 0; })
 #define huge_pte_offset(mm, address, sz)	0
@@ -234,6 +233,13 @@ static inline void __unmap_hugepage_range(struct mmu_gather *tlb,
 {
 	BUG();
 }
+static inline vm_fault_t hugetlb_fault(struct mm_struct *mm,
+				struct vm_area_struct *vma, unsigned long address,
+				unsigned int flags)
+{
+	BUG();
+	return 0;
+}
 
 #endif /* !CONFIG_HUGETLB_PAGE */
 /*