net: macb: Fix race caused by flushing unwanted descriptors

net: macb: Fix race caused by flushing unwanted descriptors

The rx descriptor list is in cached memory, and there may be multiple
descriptors per cache-line. After reclaim_rx_buffers marks a descriptor
as unused it does a cache flush, which causes the entire cache-line to
be written to memory, which may override other descriptors in the same
cache-line that the controller may have written to.

The fix skips freeing descriptors that are not the last in a cache-line,
and if the freed descriptor is the last one in a cache-line, it marks
all the descriptors in the cache-line as unused.
This is similarly to what is done in drivers/net/fec_mxc.c

In my case this bug caused tftpboot to fail some times when other
packets are sent to u-boot in addition to the ongoing tftp (e.g. ping).
The driver would stop receiving new packets because it is waiting
on a descriptor that is marked unused, when in reality the descriptor
contains a new unprocessed packet but while freeing the previous buffer
descriptor & flushing the cache, the driver accidentally marked the
descriptor as unused.

Signed-off-by: Yaron Micher <yaronm@hailo.ai>