Make rollbacks fail gracefully

Make rollbacks fail gracefully

Support for rolling back datasets require a functional ZPL, which we currently
do not have.  The zfs command does not check for ZPL support before attempting
a rollback, and in preparation for rolling back a zvol it removes the minor
node of the device.  To prevent the zvol device node from disappearing after a
failed rollback operation, this change wraps the zfs_do_rollback() function in
an #ifdef HAVE_ZPL and returns ENOSYS in the absence of a ZPL.  This is
consistent with the behavior of other ZPL dependent commands such as mount.

The orginal error message observed with this bug was rather confusing:

    internal error: Unknown error 524
    Aborted

This was because zfs_ioc_rollback() returns ENOTSUP if we don't HAVE_ZPL, but
Linux actually has no such error code.  It should instead return EOPNOTSUPP, as
that is how ENOTSUP is defined in user space.  With that we would have gotten
the somewhat more helpful message

    cannot rollback 'tank/fish': unsupported version

This is rather a moot point with the above changes since we will no longer make
that ioctl call without a ZPL.  But, this change updates the error code just in
case.

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