serial: fsl_lpuart: fix rx buffer and DMA map leaks in start_rx_dma

serial: fsl_lpuart: fix rx buffer and DMA map leaks in start_rx_dma

lpuart_start_rx_dma() allocates sport->rx_ring.buf with kzalloc() and
then maps a scatterlist via dma_map_sg().  On three subsequent error
paths the function returns directly without releasing those resources:

  - when dma_map_sg() returns 0 (-EINVAL):
      ring->buf is leaked.
  - when dmaengine_slave_config() fails:
      ring->buf and the DMA mapping are leaked.
  - when dmaengine_prep_dma_cyclic() returns NULL:
      ring->buf and the DMA mapping are leaked.

The sole cleanup path, lpuart_dma_rx_free(), is only reached when
lpuart_dma_rx_use is set, and the caller lpuart_rx_dma_startup() clears
that flag on failure of lpuart_start_rx_dma().  So these resources are
permanently leaked on every failure in this function.  Repeated port
open/close or termios changes under error conditions will slowly consume
memory and leave stale streaming DMA mappings behind.

Fix it by introducing two error labels that unmap the scatterlist and
free the ring buffer as appropriate.  While here, replace the misleading
-EFAULT (bad userspace pointer) returned when dmaengine_prep_dma_cyclic()
fails with the more accurate -ENOMEM, matching how other dmaengine users
in the tree treat this failure.

No functional change on the success path.

Fixes: 5887ad43ee02 ("tty: serial: fsl_lpuart: Use cyclic DMA for Rx")
Cc: stable <stable@kernel.org>
Signed-off-by: Shitalkumar Gandhi <shitalkumar.gandhi@cambiumnetworks.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260420135903.2062024-1-shitalkumar.gandhi@cambiumnetworks.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>