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 a pointer to struct member 'mtx_owner' in the
main struct mtx of MUTEX(9), and so we return that value.

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

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