zfs receive -> unable to handle kernel NULL pointer dereference

Fix a pair of conditions in which a concurrent umount can cause
NULl pointer dereferences:

	zfs_sb_teardown - prevent a NULL dereference by not calling
	dmu_objset_pool with a null z_os.

	zfs_resume_fs - don't try to unmount with a null z_os.	This
	change makes the ZoL code more consistent with both Illumos
	and FreeBSD.