LinuxKPI: Change flags parameter type of atomic_dec_and_lock_irqsave

On Linux atomic_dec_and_lock_irqsave is a wrapper macro which provides
a reference to third parameter rather than parameter value itself to
implementation routine called _atomic_dec_and_lock_irqsave [1].

While here, implement a fast path.

[1] https://github.com/torvalds/linux/blob/master/include/linux/spinlock.h#L476

Reviewed by: hselasky
Differential revision: https://reviews.freebsd.org/D30781