samples/kfifo: Rename read_lock/write_lock

The variables names read_lock and write_lock can clash with functions used for
read/writer locks.

Rename read_lock to read_access and write_lock to write_access to avoid a name
collision.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20210806152551.qio7c3ho6pexezup@linutronix.de
Link: https://lore.kernel.org/r/20210923172918.o22iwgvn3w7ilh44@linutronix.de
Acked-by: Stefani Seibold <stefani@seibold.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>