netconsole: prefix CPU_NR sysdata feature with SYSDATA_

Rename the CPU_NR enum value to SYSDATA_CPU_NR to establish a consistent
naming convention for sysdata features. This change prepares for
upcoming additions to the sysdata feature set by clearly grouping
related features under the SYSDATA prefix.

This change is purely cosmetic and does not modify any functionality.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>