sys/cddl/compat: Lockup mutex.h with mutex(9)

Convert FreeBSD SX(9) locks to DragonflyBSD MUTEX(9) locking.
With one caveat that an equivalent of:

 struct thread * sx_xholder(struct sx *sx);

in SX(9) is not in MUTEX(9) and so we fix the return result to be NULL

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>