cifs: populate server_hostname for extra channels

Recently, a new field got added to the smb3_fs_context struct
named server_hostname. While creating extra channels, pick up
this field from primary channel.

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