mm: convert to use vm_fault_t

As part of vm_fault_t conversion filemap_page_mkwrite() for the NOMMU case
was missed.  Now converted.

Link: http://lkml.kernel.org/r/20180828174952.GA29229@jordon-HP-15-Notebook-PCSigned-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -2748,9 +2748,9 @@ int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma)
 	return generic_file_mmap(file, vma);
 }
 #else
-int filemap_page_mkwrite(struct vm_fault *vmf)
+vm_fault_t filemap_page_mkwrite(struct vm_fault *vmf)
 {
-	return -ENOSYS;
+	return VM_FAULT_SIGBUS;
 }
 int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
 {