Remove some unneeded subroutines for padding writes to dump devices.

Right now we only need to pad when writing kernel dump headers, so
flatten three related subroutines into one. The encrypted kernel dump
code already writes out its key in a dumper.blocksize-sized block.

No functional change intended.

Reviewed by: cem, def
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D11647