mm: use helper function mapping_allow_writable()

Commit 4bb5f5d9 ("mm: allow drivers to prevent new writable mappings")
changed i_mmap_writable from unsigned int to atomic_t and add the helper
function mapping_allow_writable() to atomic_inc i_mmap_writable.  But it
forgot to use this helper function in dup_mmap() and __vma_link_file().
Signed-off-by: NMiaohe Lin <linmiaohe@huawei.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Christian Kellner <christian@kellner.me>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Adrian Reber <areber@redhat.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200917112736.7789-1-linmiaohe@huawei.comSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/fork.c

@@ -559,7 +559,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 				atomic_dec(&inode->i_writecount);
 			i_mmap_lock_write(mapping);
 			if (tmp->vm_flags & VM_SHARED)
-				atomic_inc(&mapping->i_mmap_writable);
+				mapping_allow_writable(mapping);
 			flush_dcache_mmap_lock(mapping);
 			/* insert tmp into the share list, just after mpnt */
 			vma_interval_tree_insert_after(tmp, mpnt,

mm/mmap.c

@@ -621,7 +621,7 @@ static void __vma_link_file(struct vm_area_struct *vma)
 		if (vma->vm_flags & VM_DENYWRITE)
 			atomic_dec(&file_inode(file)->i_writecount);
 		if (vma->vm_flags & VM_SHARED)
-			atomic_inc(&mapping->i_mmap_writable);
+			mapping_allow_writable(mapping);
 
 		flush_dcache_mmap_lock(mapping);
 		vma_interval_tree_insert(vma, &mapping->i_mmap);