siphash: add SPDX tags as sole licensing authority

The text "dual BSD/GPLv2 license" is somewhat ambiguous, and moving this
over to SPDX is overdue. This commit adds SPDX tags to the relevant
files and clarifies that it's GPLv2 only and 3-clause BSD. It also
removes the old text, so that the SPDX tags are the only source of the
information.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>