nfs: fixed broken compilation in nfs_callback_up_net()

Patch fixes compilation error in nfs_callback_up_net()
serv->sv_bc_enabled is defined under enabled CONFIG_SUNRPC_BACKCHANNEL,
however nfs_callback_up_net() can access it even if this config option
was not set.

Fixes: a289ce53 (sunrpc: replace svc_serv->sv_bc_xprt by boolean flag)
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>