net: sysctl: allow dump_cpumask to handle higher numbers of CPUs

net: sysctl: allow dump_cpumask to handle higher numbers of CPUs

This fixes the output of rps_default_mask and flow_limit_cpu_bitmap when
the CPU count is > 448, as it was truncated.

The underlying values are actually stored correctly when writing to
these sysctl but displaying them uses a fixed length temporary buffer in
dump_cpumask. This buffer can be too small if the CPU count is > 448.

Fix this by dynamically allocating the buffer in dump_cpumask, using a
guesstimate of what we need.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>