cifs: flush before set-info if we have writeable handles

Servers can defer destaging any data and updating the mtime until close().
This means that if we do a setinfo to modify the mtime while other handles
are open for write the server may overwrite our setinfo timestamps when
if flushes the file on close() of the writeable handle.

To solve this we add an explicit flush when the mtime is about to
be updated.

This fixes "cp -p" to preserve mtime when copying a file onto an SMB2 share.

CC: Stable <stable@vger.kernel.org>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/inode.c

@@ -2406,6 +2406,8 @@ cifs_setattr_nounix(struct dentry *direntry, struct iattr *attrs)
 	struct inode *inode = d_inode(direntry);
 	struct cifs_sb_info *cifs_sb = CIFS_SB(inode->i_sb);
 	struct cifsInodeInfo *cifsInode = CIFS_I(inode);
+	struct cifsFileInfo *wfile;
+	struct cifs_tcon *tcon;
 	char *full_path = NULL;
 	int rc = -EACCES;
 	__u32 dosattr = 0;
@@ -2452,6 +2454,20 @@ cifs_setattr_nounix(struct dentry *direntry, struct iattr *attrs)
 	mapping_set_error(inode->i_mapping, rc);
 	rc = 0;
 
+	if (attrs->ia_valid & ATTR_MTIME) {
+		rc = cifs_get_writable_file(cifsInode, false, &wfile);
+		if (!rc) {
+			tcon = tlink_tcon(wfile->tlink);
+			rc = tcon->ses->server->ops->flush(xid, tcon, &wfile->fid);
+			cifsFileInfo_put(wfile);
+			if (rc)
+				return rc;
+		} else if (rc != -EBADF)
+			return rc;
+		else
+			rc = 0;
+	}
+
 	if (attrs->ia_valid & ATTR_SIZE) {
 		rc = cifs_set_file_size(inode, attrs, xid, full_path);
 		if (rc != 0)