locking/local_lock: add localtry_trylock()

locking/local_lock: add localtry_trylock()

Add a localtry_trylock() variant without _irqsave that will be used in
slab sheaves implementation. Thanks to only disabling preemption and not
irqs, it has a lower overhead. It's not necessary to disable irqs to
avoid a deadlock if the irq context uses trylock and can handle
failures.

Also make the comment of localtry_trylock() more clear, and fix a
compilation failure in localtry_lock_init().

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>