lwIP pbuf_free() Function: Freeing pbuf Structures in Network Stack

The function `pbuf_free(p)` is used to free a `pbuf` structure in the lwIP network stack. \n\n`pbuf` is a structure that represents a data buffer in lwIP. It contains information such as the length of the buffer, the data pointer, and pointers to the next and previous `pbuf` structures in a chain.\n\nThe `pbuf_free(p)` function frees the memory occupied by the `pbuf` structure. It releases the memory back to the system so that it can be reused for other purposes. This function should be called when the `pbuf` structure is no longer needed to avoid memory leaks.\n\nNote that calling `pbuf_free(p)` only frees the memory occupied by the `pbuf` structure itself. If the `pbuf` is part of a chain of `pbuf` structures, the other `pbuf`s in the chain will not be freed. To free an entire chain of `pbuf` structures, the `pbuf_free()` function should be called on each `pbuf` in the chain.