smb: client: get rid of dfs code dep in namespace.c

Make namespace.c being built without requiring
CONFIG_CIFS_DFS_UPCALL=y by moving set_dest_addr() to dfs.c and call
it at the beginning of dfs_mount_share() so it can chase the DFS link
starting from the correct server in @ctx->dstaddr.

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