mm/mmap.c: use helper function allow_write_access() in __remove_shared_vm_struct()

In commit 1da177e4 ("Linux-2.6.12-rc2"), the helper allow_write_access
came with the atomic_inc operation of the i_writecount field in the func
__remove_shared_vm_struct().  But it forgot to use this helper function.
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/20200921115814.39680-1-linmiaohe@huawei.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -143,7 +143,7 @@ static void __remove_shared_vm_struct(struct vm_area_struct *vma,
 		struct file *file, struct address_space *mapping)
 {
 	if (vma->vm_flags & VM_DENYWRITE)
-		atomic_inc(&file_inode(file)->i_writecount);
+		allow_write_access(file);
 	if (vma->vm_flags & VM_SHARED)
 		mapping_unmap_writable(mapping);