bpf: fix possible spectre-v1 in find_and_alloc_map()

bpf: fix possible spectre-v1 in find_and_alloc_map()

It's possible for userspace to control attr->map_type. Sanitize it when
using it as an array index to prevent an out-of-bounds value being used
under speculation.

Found by smatch.

Signed-off-by: Mark Rutland <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Dan Carpenter <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Acked-by: David S. Miller <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>