sw64: kvm: fix incorrect page_ref_count() call

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I56WV8



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The last page_ref_count() call will cause kernel panic if
kvm memory pool is at the end of DRAM. This patch reorders
the checks to avoid illegal atomic_read operation.

Signed-off-by: Min Fanlei <minfanlei@wxiat.com>

Signed-off-by: Gu Zitao <guzitao@wxiat.com>