iommu/amd: Convert from atomic_t to refcount_t on pasid_state->count

iommu/amd: Convert from atomic_t to refcount_t on pasid_state->count

refcount_t type and corresponding API can protect refcounters from
accidental underflow and overflow and further use-after-free situations.

Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/1626683578-64214-1-git-send-email-xiyuyang19@fudan.edu.cn
Signed-off-by: Joerg Roedel <jroedel@suse.de>