NFS: return DENIED in decode_lock_denied if we cannot decode owner

After commit 43502f6e8d ("NFS: fix open_owner_id_maxsz and related
fields.") we dramatically changed the size of the LOCK/LOCKT reply buffer
from 164 to 52 bytes. This made visible a situation on a buggy NFS
server that sent oversized denied responses which led to us returning
EIO instead of DENIED.

Now, apart from buggy server issue, this poses an interesting question:
what if other implementations have legitimate but bigger than we expect
lock owner response, especially now that we have reduced the size we
allocate for it (20 bytes for the lock owner part).

This change proposes to tackle that issue returning DENIED instead of
EIO regardless of whether we manage to decode the owner or not:

- At this point of decode_lock_denied we know there is an owner.
- Server legitimately denied the lock request, with an owner that fits on
  NFS4_OPAQUE_LIMIT but we returned EIO instead to userspace.
- The owner data is decoded but actually never used, decode_lock path
  simply retries, and decode_lockt turns it into 0 on file_lock and
  returns it to userspace.
- LOCK/LOCKT are the last operations on the compound so it's not relevant
  if we didn't advance the pointer.
- Also removes an inverted likely(!p) hint.

Fixes: 43502f6e8d ("NFS: fix open_owner_id_maxsz and related fields.")
Signed-off-by: Roberto Bergantinos Corpas <rbergant@redhat.com>
Reviewed-by: Prabhakar Pujeri <prabhakar.pujeri@dell.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>

fs/nfs/nfs4xdr.c

@@ -5169,8 +5169,7 @@ static int decode_lock_denied(struct xdr_stream *xdr, struct file_lock *fl)
 	p = xdr_decode_hyper(p, &clientid); /* read 8 bytes */
 	namelen = be32_to_cpup(p); /* read 4 bytes */  /* have read all 32 bytes now */
 	p = xdr_inline_decode(xdr, namelen); /* variable size field */
-	if (likely(!p))
-		return -EIO;
+	/* We have an owner here, return DENIED */
 	return -NFS4ERR_DENIED;
 }