[CIFS] Mount should fail if server signing off but client mount option requires it

Currently, if mount with a signing-enabled sec= option (e.g.
sec=ntlmi), the kernel does a warning printk if the server doesn't
support signing, and then proceeds without signatures.

This is probably OK for people that think to look at the ring buffer,
but seems wrong to me. If someone explicitly requests signing, we
should error out if that request can't be satisfied. They can then
reattempt the mount without signing if that's ok.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/CHANGES

@@ -12,6 +12,7 @@ to match what documentation said. Support for very large reads, over 127K,
 available to some newer servers (such as Samba 3.0.26 and later but
 note that it also requires setting CIFSMaxBufSize at module install
 time to a larger value which may hurt performance in some cases).
+Make sign option force signing (or fail if server does not support it).
 
 Version 1.48
 ------------

fs/cifs/cifssmb.c

@@ -645,11 +645,12 @@ signing_check:
 			~(SECMODE_SIGN_ENABLED | SECMODE_SIGN_REQUIRED);
 	} else if ((secFlags & CIFSSEC_MUST_SIGN) == CIFSSEC_MUST_SIGN) {
 		/* signing required */
-		cFYI(1, ("Must sign - segFlags 0x%x", secFlags));
+		cFYI(1, ("Must sign - secFlags 0x%x", secFlags));
 		if ((server->secMode &
 			(SECMODE_SIGN_ENABLED | SECMODE_SIGN_REQUIRED)) == 0) {
 			cERROR(1,
 				("signing required but server lacks support"));
+			rc = -EOPNOTSUPP;
 		} else
 			server->secMode |= SECMODE_SIGN_REQUIRED;
 	} else {