NFS: move NFS_DEBUG definition

Trond, looks like the changes to include/linux/nfs_fs.h in 2.6.18
that moved the #include's of sunrpc header files into the #ifdef __KERNEL__ block
disabled nfs debugging for all nfs c file not including any sunrpc header.

The following patch moves the definition down, right before its use
for defining ifdebug.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
(Moved definition further down into the __KERNEL__ section: Trond)
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>