ANDROID: KVM: Do not memset hyp_page from reclaim_hyp_pool()

reclaim_hyp_pool() zeroes the entire struct hyp_page when returning a
page to the host. However, this is no longer the canonical way to do it
since we store the host ownership state in struct hyp_page.

Fix this by only changing the refcount and order members of the
hyp_page, and while at it make the code common with drain_hyp_pool()
which does something very similar.

Bug: 379022162
Change-Id: I58941ae6ef8e62ff17cba8508cb41cddfefebe1d
Signed-off-by: Quentin Perret <qperret@google.com>