bpf, mprog: Fix maximum program check on mprog attachment

After Paul's recent improvement to syzkaller to improve coverage for
bpf_mprog and tcx, it hit a splat that the program limit was surpassed.
What happened is that the maximum number of progs got added, followed
by another prog add request which adds with BPF_F_BEFORE flag relative
to the last program in the array. The idx >= bpf_mprog_max() check in
bpf_mprog_attach() still passes because the index is below the maximum
but the maximum will be surpassed. We need to add a check upfront for
insertions to catch this situation.

Fixes: 053c8e1f235d ("bpf: Add generic attach/detach/query API for multi-progs")
Reported-by: [email protected]
Reported-by: [email protected]
Reported-by: [email protected]
Co-developed-by: Nikolay Aleksandrov <[email protected]>
Signed-off-by: Nikolay Aleksandrov <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Tested-by: [email protected]
Tested-by: [email protected]
Link: https://github.com/google/syzkaller/pull/4207
Link: https://lore.kernel.org/bpf/[email protected]

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