net: inline skb_add_rx_frag_netmem()

net: inline skb_add_rx_frag_netmem()

This critical helper (via skb_add_rx_frag()) is mostly used
from drivers rx fast path.

It is time to inline it, this actually saves space in vmlinux:

size vmlinux.old vmlinux
   text    data     bss     dec     hex filename
37350766        23092977        4846992 65290735        3e441ef vmlinux.old
37350600        23092977        4846992 65290569        3e44149 vmlinux

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260226041213.1892561-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>