arm64/gcs: Don't call gcs_free() during flush_gcs()

Currently we call gcs_free() during flush_gcs() to reset the thread state
for GCS. This includes unmapping any kernel allocated GCS, but this is
redundant when doing a flush_thread() since we are reinitialisng the thread
memory too. Inline the reinitialisaton of the thread struct.

Signed-off-by: Mark Brown <broonie@kernel.org>