Fix zfs_sb_teardown/zfs_resume_fs NULL 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.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1543