snapshot: use local variable to bdrv_pwrite_sync L1 table

Use local variable to bdrv_pwrite_sync L1 table,
needless to make conversion of cached L1 table between
big-endian and host style.

Signed-off-by: Zhang Haoyu <zhanghy@sangfor.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>