serial: qcom_geni: fix kfifo underflow when flush precedes DMA completion IRQ

In the Linux kernel, the following vulnerability has been resolved:

serial: qcom_geni: fix kfifo underflow when flush precedes DMA completion IRQ

When uart_flush_buffer() runs before the DMA completion IRQ is delivered,

the following race can occur (all steps serialized by uart_port_lock):

1. DMA starts: tx_remaining = N, kfifo contains N bytes

2. DMA completes in hardware; IRQ is pending but not yet delivered

3. uart_flush_buffer() acquires the port lock and calls kfifo_reset(),

making kfifo_len() = 0 while tx_remaining remains N

4. uart_flush_buffer() releases the port lock

5. DMA IRQ fires; handle_tx_dma() acquires the port lock and calls

uart_xmit_advance(uport, tx_remaining) on an empty kfifo

uart_xmit_advance() increments kfifo->out by tx_remaining. Since

kfifo_reset() already set both in and out to 0, out wraps past in,

causing kfifo_len() to return UART_XMIT_SIZE - tx_remaining. The next

start_tx_dma() call then submits a DMA transfer of stale buffer data.

Fix this by snapshotting kfifo_len() at the start of handle_tx_dma()

and skipping uart_xmit_advance() when fifo_len < tx_remaining, which

indicates the kfifo was reset by a preceding flush.