NFS: Fix a compile error when !defined NFS_DEBUG

We should use the 'ifdebug' wrapper rather than trying to inline
tests of nfs_debug, so that the code compiles correctly when we
don't define NFS_DEBUG.

Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>