eth: fbnic: Fix counter roll-over issue

Fix a potential counter roll-over issue in fbnic_mbx_alloc_rx_msgs()
when calculating descriptor slots. The issue occurs when head - tail
results in a large positive value (unsigned) and the compiler interprets
head - tail - 1 as a signed value.

Since FBNIC_IPC_MBX_DESC_LEN is a power of two, use a masking operation,
which is a common way of avoiding this problem when dealing with these
sort of ring space calculations.

Fixes: da3cde08209e ("eth: fbnic: Add FW communication mechanism")
Signed-off-by: Mohsin Bashir <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/meta/fbnic/fbnic_fw.c

···
 		return -ENODEV;
 
 	/* Fill all but 1 unused descriptors in the Rx queue. */
-	count = (head - tail - 1) % FBNIC_IPC_MBX_DESC_LEN;
+	count = (head - tail - 1) & (FBNIC_IPC_MBX_DESC_LEN - 1);
 	while (!err && count--) {
 		struct fbnic_tlv_msg *msg;