nfsd/sunrpc: move pool_mode definitions into svc.h

In later patches, we're going to need to allow code external to svc.c
to figure out what pool_mode is in use. Move these definitions into
svc.h to prepare for that.

Also, make the svc_pool_map object available and exported so that other
modules can peek in there to get insight into what pool mode is in use.
Likewise, export svc_pool_map_get/put function to make it safe to do so.

Signed-off-by: Shirley Ma <shirley.ma@oracle.com>
Acked-by: Jeff Layton <jlayton@primarydata.com>
Tested-by: Shirley Ma <shirley.ma@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>