rds: rds_rm_zerocopy_callback() use list_first_entry()

rds_rm_zerocopy_callback() uses list_entry() on the head of a list
causing a type confusion.
Use list_first_entry() to actually access the first element of the
rs_zcookie_queue list.

Fixes: 9426bbc6de99 ("rds: use list structure to track information for zerocopy completion notification")
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
Link: https://lore.kernel.org/r/20230202-rds-zerocopy-v3-1-83b0df974f9a@diag.uniroma1.it
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
