xsk: move xsk_tx_metadata_request() to xdp_sock_drv.h

xsk: move xsk_tx_metadata_request() to xdp_sock_drv.h

xsk_tx_metadata_request() must validate metadata with
xsk_buff_valid_tx_metadata(), which is defined in xdp_sock_drv.h. Move the
helper there before adding that dependency. All callers already include
the destination header, so this has no functional effect.

Fixes: ca4419f15abd ("xsk: Add launch time hardware offload support to XDP Tx metadata")
Cc: Cen Zhang (Microsoft) <blbllhy@gmail.com>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260727161959.885642-6-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>