NFSv4: Fix the sync mount option for nfs4 mounts

The sync mount option stopped working for NFSv4 mounts after commit
c02d7adf8c5429727a98bad1d039bccad4c61c50 (NFSv4: Replace nfs4_path_walk() with
FS path lookup in a private namespace). If MS_SYNCHRONOUS is set in the
super_block that we're cloning from, then it should be set in the new
super_block as well.

Signed-off-by: Scott Mayhew <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/super.c

···
 	if (server->flags & NFS_MOUNT_NOAC)
 		sb_mntdata.mntflags |= MS_SYNCHRONOUS;
 
+	if (mount_info->cloned != NULL && mount_info->cloned->sb != NULL)
+		if (mount_info->cloned->sb->s_flags & MS_SYNCHRONOUS)
+			sb_mntdata.mntflags |= MS_SYNCHRONOUS;
+
 	/* Get a superblock - note that we may end up sharing one that already exists */
 	s = sget(nfs_mod->nfs_fs, compare_super, nfs_set_super, flags, &sb_mntdata);
 	if (IS_ERR(s)) {