Fix memory leaks in cifs_do_mount()

Fix memory leaks introduced by the patch
fs/cifs: make share unaccessible at root level mountable

Also move allocation of cifs_sb->prepath to cifs_setup_cifs_sb().

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Tested-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <smfrench@gmail.com>