sock: Introduce sk_set_prio_allowed helper function

sock: Introduce sk_set_prio_allowed helper function

Simplify priority setting permissions with the 'sk_set_prio_allowed'
function, centralizing the validation logic. This change is made in
anticipation of a second caller in a following patch.
No functional changes.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Anna Emese Nyiri <annaemesenyiri@gmail.com>
Link: https://patch.msgid.link/20241213084457.45120-2-annaemesenyiri@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>