sstatesig: Avoid resetting taskhash within siggen for locked sigs

sstatesig: Avoid resetting taskhash within siggen for locked sigs

Since get_unihash uses taskhash as a key internally, changing it means
different bebahour when locked sigs are active verses not active. Under
corner cases this leads to a signature mismatch.

Avoid this by by adding a wrapper for the place its externally exposed
and then not changing the internals.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>