netconsole: Include sysdata in extradata entry count

Modify count_extradata_entries() to include sysdata fields when
calculating the total number of extradata entries. This change ensures
that the sysdata feature, specifically the CPU number field, is
correctly counted against the MAX_EXTRADATA_ITEMS limit.

The modification adds a simple check for the CPU_NR flag in the
sysdata_fields, incrementing the entry count accordingly.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>