Give ENOTSUP a valid user space error value

Give ENOTSUP a valid user space error value

The ZFS module returns ENOTSUP for several error conditions where an operation
is not (yet) supported.  The SPL defined ENOTSUP in terms of ENOTSUPP, but that
is an internal Linux kernel error code that should not be seen by user
programs.  As a result the zfs utilities print a confusing error message if an
unsupported operation is attempted:

    internal error: Unknown error 524
    Aborted

This change defines ENOTSUP in terms of EOPNOTSUPP which is consistent with
user space.

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