MFC r343346

netmap: improvements to the netmap kloop (CSB mode)

Changelist:

• Add the proper memory barriers in the kloop ring processing functions.
• Fix memory barriers usage in the user helpers (nm_sync_kloop_appl_write, nm_sync_kloop_appl_read).
• Fix nm_kr_txempty() helper to look at rhead rather than rcur. This is important since the kloop can read a value of rcur which is ahead of the value of rhead (see explanation in nm_sync_kloop_appl_write)
• Remove obsolete ptnetmap_guest_write_kring_csb() and ptnet_guest_read_kring_csb(), and update if_ptnet(4) to use those.
• Prepare in advance the arguments for netmap_sync_kloop_[tr]x_ring(), to make the kloop faster.
• Provide kernel and user implementation for nm_ldld_barrier() and nm_ldst_barrier()