bpf: check max_entries before allocating memory

bpf: check max_entries before allocating memory

For maps of type BPF_MAP_TYPE_CPUMAP memory is allocated first before
checking the max_entries argument. If then max_entries is greater than
NR_CPUS additional work needs to be done to free allocated memory before
an error is returned.
This changes moves the check on max_entries before the allocation
happens.

Signed-off-by: Florian Lehner <dev@der-flo.net>
Link: https://lore.kernel.org/r/20221028183405.59554-1-dev@der-flo.net
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>