Improvements to set_user_ldt().

Remove mtx_owned() checks from set_user_ldt(). Split the function
into _locked() version which requires the dt_lock spinlock owned, and
make set_user_ldt() a wrapper. Add a comment in swtch.s noting that
the call to the new set_user_ldt() cannot recurse on dt_lock.

Remove #ifdef SMP block, the addend is always zero on UP.

Fix type of set_user_ldt_rv(), making it match the type used for
smb_rendezvous() callback, and remove the cast. Use curproc.

Reviewed by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week