net: xilinx: axienet: Add error handling for RX metadata pointer retrieval

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

net: xilinx: axienet: Add error handling for RX metadata pointer retrieval

Add proper error checking for dmaengine_desc_get_metadata_ptr() which can return an error pointer and lead to potential crashes or undefined behaviour if the pointer retrieval fails.

Properly handle the error by unmapping DMA buffer, freeing the skb and returning early to prevent further processing with invalid data.