cifs: don't block in dfs_cache_noreq_update_tgthint()

Avoid blocking in dfs_cache_noreq_update_tgthint() while reconnecting
servers or tcons as the cache refresh worker or new mounts might
already be updating their targets.

Move some more dfs related code out of connect.c while at it.

Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>