zfs: fix EIO accessing dataset after resuming interrupted receive
ZFS unmounts a dataset while receiving into it and remounts it afterwards.
But if ZFS is resuming an incomplete receive, it screws up and ends up with
a dataset that is mounted, but returns EIO for every access. This commit
fixes that condition.
While the vulnerable code also exists in OpenZFS, the problem is not
reproducible there. Apparently OpenZFS doesn't unmount the destination
dataset during receive, like FreeBSD does.