Fix netdump buffering after r348473.

nd_buf is used to buffer headers (for both the kernel dump itself and
for EKCD) before the final call to netdump_dumper(), which flushes
residual data in nd_buf. As a result, a small portion of the residual
data would be corrupted. This manifests when kernel dump compression
is enabled since both zstd and zlib detect the corruption during
decompression.

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D21294