NFS: Fix show_nfs_errors macros again

I noticed that NFS status values stopped working again.

trace_print_symbols_seq() takes an unsigned long. Passing a negative
errno or negative NFSERR value just confuses it, and since we're
using C macros here and not static inline functions, all bets are
off due to implicit type conversion.

Straight-line the calling conventions so that error codes are stored
in the trace record as positive values in an unsigned long field,
mapped to symbolic as an unsigned long, and displayed as a negative
value, to continue to enable grepping on "error=-".

It's often the case that an error value that is positive is a byte
count but when it's negative, it's an error (e.g. nfs4_write). Fix
those cases so that the value that is eventually stored in the
error field is a positive NFS status or errno, or zero.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>