net: stmmac: fix transmit interrupt coalescing

net: stmmac: fix transmit interrupt coalescing

The accounting for transmit frames does not count the descriptors
correctly. It uses:

	tx_packets = (tx_q->cur_tx + 1) - first_tx;

however, these are indexes into a circular buffer, so cur_tx can be
less than first_tx, and when that happens, tx_packets becomes a very
large unsigned integer. When this is added to tx_q->tx_count_frames,
it has the effect of reducing the count of frames, possibly causing
it to also wrap to a very large unsigned integer.

Fix this by using CIRC_CNT() to calculate the number of descriptors
used.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/E1vuoIl-0000000Aouz-0ttb@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>