First in a series of changes to remove the now-unused Giant compatibility
framework for non-MPSAFE network protocols:

• Remove debug_mpsafenet variable, sysctl, and tunable.
• Remove NET_NEEDS_GIANT() and associate SYSINITSs used by it to force debug.mpsafenet=0 if non-MPSAFE protocols are compiled into the kernel.
• Remove logic to automatically flag interrupt handlers as non-MPSAFE if debug.mpsafenet is set for an INTR_TYPE_NET handler.
• Remove logic to automatically flag netisr handlers as non-MPSAFE if debug.mpsafenet is set.
• Remove references in a few subsystems, including NFS and Cronyx drivers, which keyed off debug_mpsafenet to determine various aspects of their own locking behavior.
• Convert NET_LOCK_GIANT(), NET_UNLOCK_GIANT(), and NET_ASSERT_GIANT into no-op's, as their entire behavior was determined by the value in debug_mpsafenet.
• Alias NET_CALLOUT_MPSAFE to CALLOUT_MPSAFE.

Many remaining references to NET_.*_GIANT() and NET_CALLOUT_MPSAFE are still
present in subsystems, and will be removed in followup commits.

Reviewed by: bz, jhb
Approved by: re (kensmith)