Remove Giant locking from domains list.
Remove Giant locking from domains list.

During boot, the domain list is locked with Giant. It is not possible to
register any protocols after the system has booted, so the lock is only
used to protect insertion of entries.

There is already a mutex in uipc_domain.c called dom_mtx. Use this mutex
to lock the list, instead of using Giant. It won't matter anything with
respect to performance, but we'll never get rid of Giant if we don't
remove from places where we don't need it.

Approved by:	rwatson
MFC after:	3 weeks

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