NFS: set varaiable nfs_netfs_debug_id storage-class-specifier to static

smatch reports
fs/nfs/fscache.c:260:10: warning: symbol
  'nfs_netfs_debug_id' was not declared. Should it be static?

This variable is only used in its defining file, so it should be static

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>