net: rename dsa_realloc_skb to skb_ensure_writable_head_tail

Rename dsa_realloc_skb to skb_ensure_writable_head_tail and move it to
skbuff.c to use it as helper.

Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@oss.nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>