smb3: snapshot mounts are read-only and make sure info is displayable about the mount

snapshot mounts were not marked as read-only and did not display the snapshot
time (in /proc/mounts) specified on mount

With this patch - note that can not write to the snapshot mount (see "ro" in
/proc/mounts line) and also the missing snapshot timewarp token time is
dumped.  Sample line from /proc/mounts with the patch:

//127.0.0.1/scratch /mnt2 smb3 ro,relatime,vers=default,cache=strict,username=testuser,domain=,uid=0,noforceuid,gid=0,noforcegid,addr=127.0.0.1,file_mode=0755,dir_mode=0755,soft,nounix,serverino,mapposix,noperm,rsize=1048576,wsize=1048576,echo_interval=60,snapshot=1234567,actimeo=1 0 0
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Paulo Alcantara <palcantara@suse.de>

fs/cifs/cifsfs.c

@@ -139,6 +139,9 @@ cifs_read_super(struct super_block *sb)
 	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_POSIXACL)
 		sb->s_flags |= SB_POSIXACL;
 
+	if (tcon->snapshot_time)
+		sb->s_flags |= SB_RDONLY;
+
 	if (tcon->ses->capabilities & tcon->ses->server->vals->cap_large_files)
 		sb->s_maxbytes = MAX_LFS_FILESIZE;
 	else
@@ -540,6 +543,8 @@ cifs_show_options(struct seq_file *s, struct dentry *root)
 	seq_printf(s, ",wsize=%u", cifs_sb->wsize);
 	seq_printf(s, ",echo_interval=%lu",
 			tcon->ses->server->echo_interval / HZ);
+	if (tcon->snapshot_time)
+		seq_printf(s, ",snapshot=%llu", tcon->snapshot_time);
 	/* convert actimeo and display it in seconds */
 	seq_printf(s, ",actimeo=%lu", cifs_sb->actimeo / HZ);