CIFS: Fix oplock handling for SMB 2.1+ protocols

There may be situations when a server negotiates SMB 2.1
protocol version or higher but responds to a CREATE request
with an oplock rather than a lease.

Currently the client doesn't handle such a case correctly:
when another CREATE comes in the server sends an oplock
break to the initial CREATE and the client doesn't send
an ack back due to a wrong caching level being set (READ
instead of RWH). Missing an oplock break ack makes the
server wait until the break times out which dramatically
increases the latency of the second CREATE.

Fix this by properly detecting oplocks when using SMB 2.1
protocol version and higher.

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

fs/cifs/smb2ops.c

@@ -3333,6 +3333,11 @@ smb21_set_oplock_level(struct cifsInodeInfo *cinode, __u32 oplock,
 	if (oplock == SMB2_OPLOCK_LEVEL_NOCHANGE)
 		return;
 
+	/* Check if the server granted an oplock rather than a lease */
+	if (oplock & SMB2_OPLOCK_LEVEL_EXCLUSIVE)
+		return smb2_set_oplock_level(cinode, oplock, epoch,
+					     purge_cache);
+
 	if (oplock & SMB2_LEASE_READ_CACHING_HE) {
 		new_oplock |= CIFS_CACHE_READ_FLG;
 		strcat(message, "R");