sa_lookup() ignores buffer size.

When retrieving a system attribute, the size of the supplied
buffer is ignored. If the buffer is too small to hold the attribute,
sa_attr_op() will write past the end of the buffer.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jason King <jking@racktopsystems.com>
Closes #15476