fix bug after introduce "make ktap reentrant" patch

System will hang when using command:
./ktap -e 'trace "*:*" function (e) { print(execname(), e }'

The reason is execname() will dynamic allocate memory, and
operate global string table, but it's only under protect
by spin_lock/spin_unlock, this is safe when ktap handler is running
in irq disabled, but after ktap make change to reentrant,
then spin_lock/spin_unlock is not safe anymore, change it to
spin_lock_irqsave/spin_unlock_irqrestore