fix(tx_pool): avoid pointer arithmetic on a void * in homa_copy_iter_to_frags

[randomizedcoder]

Bug: pointer arithmetic on a void * in homa_copy_iter_to_frags()

u8 *vaddr = kmap_local_page(p);        /* sibling homa_copy_to_frags(): u8 * */
...
void *vaddr = kmap_local_page(p);      /* homa_copy_iter_to_frags(): void * */
result = copy_from_iter(vaddr + p_off, p_len, iter);   /* void* arithmetic */

vaddr + p_off on a void * is a GNU extension and undefined in standard C. The sibling function homa_copy_to_frags() a few lines up already declares its mapping as u8 *vaddr; this just makes the two consistent.

Fix

-			void *vaddr = kmap_local_page(p);
+			u8 *vaddr = kmap_local_page(p);

Verification (static gate + regression)

Before:  homa_tx_pool.c:534: portability: 'vaddr' is of type 'void *'. When using
         void pointers in calculations, the behaviour is undefined.
         [arithOperationsOnVoidPointer]
After:   (clean — finding gone)

cppcheck --enable=all homa_tx_pool.c. The existing homa_copy_iter_to_frags and homa_copy_to_frags unit tests both stay green (2/2) — the byte-copy behaviour is unchanged (u8 * and the previous void * arithmetic address the same bytes).