Use atomic_fence_fence_rel() to ensure ordering in the
seq_write_begin(), instead of the load_rmb/rbm_load functions. The
update does not need to be atomic due to the write lock owned.

Similarly, in seq_write_end(), update of *seqp needs not be atomic.
Only store must be atomic with release.

For seq_read(), the natural operation is the load acquire of the
sequence value, express this directly with atomic_load_acq_int()
instead of using custom partial fence implementation
atomic_load_rmb_int().

In seq_consistent, use atomic_thread_fence_acq() which provides the
desired semantic of ordering reads before fence before the re-reading
of *seqp, instead of custom atomic_rmb_load_int().

Reviewed by: alc, bde
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks