ANDROID: bring back kfree_skb()

In commit 5158e182 ("net: skb: introduce kfree_skb_reason()"),
kfree_skb() was turned into an inline function calling
kfree_skb_reason() instead.  To preserve the Android ABI, turn it back
into a "real" function so that external drivers can still find this
symbol.

Bug: 161946584
Fixes: 5158e182 ("net: skb: introduce kfree_skb_reason()")
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I0f54443c0793bb29e847be5921f53e09ca2981ad