mmap_region: cleanup the final vma_merge() related code

It is not easy to actually understand the "if (!file || !vma_merge())"
code, turn it into "if (file && vma_merge())".  This makes immediately
obvious that the subsequent "if (file)" is superfluous.

As Hugh Dickins pointed out, we can also factor out the ->i_writecount
corrections, and add a small comment about that.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -1068,7 +1068,6 @@ int vma_wants_writenotify(struct vm_area_struct *vma)
 		mapping_cap_account_dirty(vma->vm_file->f_mapping);
 }
 
-
 unsigned long mmap_region(struct file *file, unsigned long addr,
 			  unsigned long len, unsigned long flags,
 			  unsigned int vm_flags, unsigned long pgoff,
@@ -1181,21 +1180,19 @@ munmap_back:
 	if (vma_wants_writenotify(vma))
 		vma->vm_page_prot = vm_get_page_prot(vm_flags & ~VM_SHARED);
 
-	if (!file || !vma_merge(mm, prev, addr, vma->vm_end,
+	if (file && vma_merge(mm, prev, addr, vma->vm_end,
 			vma->vm_flags, NULL, file, pgoff, vma_policy(vma))) {
-		file = vma->vm_file;
-		vma_link(mm, vma, prev, rb_link, rb_parent);
-		if (correct_wcount)
-			atomic_inc(&inode->i_writecount);
-	} else {
-		if (file) {
-			if (correct_wcount)
-				atomic_inc(&inode->i_writecount);
-			fput(file);
-		}
 		mpol_free(vma_policy(vma));
 		kmem_cache_free(vm_area_cachep, vma);
+		fput(file);
+	} else {
+		vma_link(mm, vma, prev, rb_link, rb_parent);
+		file = vma->vm_file;
 	}
+
+	/* Once vma denies write, undo our temporary denial count */
+	if (correct_wcount)
+		atomic_inc(&inode->i_writecount);
 out:
 	mm->total_vm += len >> PAGE_SHIFT;
 	vm_stat_account(mm, vm_flags, file, len >> PAGE_SHIFT);