uart_putxmitchar() manipulates dev->xmit.head/buffer directly with
no internal locking - every other caller (uart_write()) takes
dev->xmit.lock first. uart_readv()'s ECHO handling (both the
backspace/delete erase sequence and the normal character echo)
calls uart_putxmitchar() without taking that lock, so a concurrent
uart_write() and a local echo can race on the same circular buffer
state, corrupting it.
Take dev->xmit.lock around each echo's uart_putxmitchar() calls,
matching what uart_write() already does. uart_readv() holds
dev->recv.lock for its own duration, but no other code path ever
acquires recv.lock while holding xmit.lock, so nesting xmit.lock
inside the existing recv.lock scope here doesn't introduce a new
lock-ordering cycle.
Fixes#14845
Signed-off-by: yi chen <94xhn1@gmail.com>
6fdc2dde41