net: skb: clean up dead code after skb_kfree_head() simplification

net: skb: clean up dead code after skb_kfree_head() simplification

Since commit 0f42e3f4fe2a ("net: skb: fix cross-cache free of
KFENCE-allocated skb head"), skb_kfree_head() always calls kfree()
and no longer uses end_offset to distinguish between skb_small_head_cache
and generic kmalloc caches.

Clean up the leftovers:

- Remove the unused end_offset parameter from skb_kfree_head() and
  update all callers.
- Remove the SKB_SMALL_HEAD_HEADROOM guard in __skb_unclone_keeptruesize()
  which was protecting the old skb_kfree_head() logic.
- Update the SKB_SMALL_HEAD_CACHE_SIZE comment to reflect that the
  non-power-of-2 sizing is no longer used for free-path disambiguation.

No functional change.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260410034736.297900-1-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>