bpf: convert __bpf_prog_get() to CLASS(fd, ...)

Irregularity here is fdput() not in the same scope as fdget();
just fold ____bpf_prog_get() into its (only) caller and that's
it...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>