unless it's the old 4-clause BSD license (with the advertising clause,
which almost nobody used anymore), the BSD code can be relicensed as
anything (including proprietary), as long as the notices in the file are
maintained.
it doesn't take more explicit permission to re-license something as GPL
than it does to make it closed-source.
However,
there are quite a few files in the kernel that are BSD licensed, when
combined with other GPL code, the only way you can re-distribute the
result is under the GPL, so it is effectivly 'converted' when you compile,
but by leaving the file BSD, improvements to it can be shared back with
the original authors and put into their main codebase, so it's actually
more polite to leave the license as-is for this file.
David Lang
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