Remove spaces from CTL devices' default serial numbers

It's awkward to have spaces in CAM device serial numbers. That leads to
such things as device nodes named "/dev/diskid/MYSERIAL%20%20%201". Better
to replace the spaces with "0"s. This change only affects the default
serial numbers for users who don't provide their own.

Reviewed by: ken, mav
MFC after: Never
Relnotes: Yes
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D12263