zfs_mount(MS_REMOUNT): protect zfs_(un)register_callbacks calls
We now take z_teardown_lock as a writer to ensure that there is no I/O
while the filesystem state is in a flux. Also, zfs_suspend_fs() ->
zfsvfs_teardown() call zfs_unregister_callbacks() and zfs_resume_fs() ->
zfsvfs_setup() call zfs_unregister_callbacks(). Previously there was no
synchronization between those calls and the calls in the re-mounting
case. That could lead to concurrent execution and a crash.