smb3: fix redundant opens on root

In SMB2/SMB3 unlike in cifs we unnecessarily open the root of the share
over and over again in various places during mount and path revalidation
and also in statfs.  This patch cuts redundant traffic (opens and closes)
by simply keeping the directory handle for the root around (and reopening
it as needed on reconnect), so query calls don't require three round
trips to copmlete - just one, and eases load on network, client and
server (on mount alone, cuts network traffic by more than a third).

Also add a new cifs mount parm "nohandlecache" to allow users whose
servers might have resource constraints (eg in case they have a server
with so many users connecting to it that this extra handle per mount
could possibly be a resource concern).

Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>

fs/cifs/cifssmb.c

@@ -106,6 +106,12 @@ cifs_mark_open_files_invalid(struct cifs_tcon *tcon)
 		open_file->oplock_break_cancelled = true;
 	}
 	spin_unlock(&tcon->open_file_lock);
+
+	mutex_lock(&tcon->prfid_mutex);
+	tcon->valid_root_fid = false;
+	memset(tcon->prfid, 0, sizeof(struct cifs_fid));
+	mutex_unlock(&tcon->prfid_mutex);
+
 	/*
 	 * BB Add call to invalidate_inodes(sb) for all superblocks mounted
 	 * to this tcon.