NFS: Fix fix of show_nfs_errors

Casting a negative value to an unsigned long is not the same as
converting it to its absolute value.

Fixes: 96650e2e ("NFS: Fix show_nfs_errors macros again")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>