plasma_spin_tktlock and plasma_spin_taglock

plasma_spin_tktlock - ticket lock implementation (fair spinlock)

plasma_spin_taglock - ticket lock batching by tag (experimental)
At least with current implementation, tktlock seems superior to taglock
unless there are more simultaneous contenders for lock than there are
CPU cores.  (If that happens, should probably revisit thread work
distribution instead of using taglock)