ext2: don't update mtime on COW faults

When running in a dax mode, if the user maps a page with MAP_PRIVATE and
PROT_WRITE, the ext2 filesystem would incorrectly update ctime and mtime
when the user hits a COW fault.

This breaks building of the Linux kernel.  How to reproduce:

 1. extract the Linux kernel tree on dax-mounted ext2 filesystem
 2. run make clean
 3. run make -j12
 4. run make -j12

at step 4, make would incorrectly rebuild the whole kernel (although it
was already built in step 3).

The reason for the breakage is that almost all object files depend on
objtool.  When we run objtool, it takes COW page fault on its .data
section, and these faults will incorrectly update the timestamp of the
objtool binary.  The updated timestamp causes make to rebuild the whole
tree.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ext2/file.c

@@ -93,8 +93,10 @@ static vm_fault_t ext2_dax_fault(struct vm_fault *vmf)
 	struct inode *inode = file_inode(vmf->vma->vm_file);
 	struct ext2_inode_info *ei = EXT2_I(inode);
 	vm_fault_t ret;
+	bool write = (vmf->flags & FAULT_FLAG_WRITE) &&
+		(vmf->vma->vm_flags & VM_SHARED);
 
-	if (vmf->flags & FAULT_FLAG_WRITE) {
+	if (write) {
 		sb_start_pagefault(inode->i_sb);
 		file_update_time(vmf->vma->vm_file);
 	}
@@ -103,7 +105,7 @@ static vm_fault_t ext2_dax_fault(struct vm_fault *vmf)
 	ret = dax_iomap_fault(vmf, PE_SIZE_PTE, NULL, NULL, &ext2_iomap_ops);
 
 	up_read(&ei->dax_sem);
-	if (vmf->flags & FAULT_FLAG_WRITE)
+	if (write)
 		sb_end_pagefault(inode->i_sb);
 	return ret;
 }