Cleanup: Remove constant comparisons reported by CodeQL

CodeQL's cpp/constant-comparison query from its security-and-extended
query set reported 4 instances where we have comparions that always
evaluate the same way.

In draid_config_by_type(), we have an early if (nparity == 0) check
that returns EINVAL, making a later `if (nparity == 0 || nparity >
VDEV_DRAID_MAXPARITY)` partially redundant. The later check prints an
error message when parity is 0, but the early check does not. This is
not useful feedback, so we move the later check to the place where the
early check runs to replace the early check.

In perform_thread_merge(), we return when num_threads == 0. After
that block, we do if (num_threads > 0) {, which will always be true.
We remove the if statement.

In sa_modify_attrs(), we have a loop condition that is k != 2, but
at the end of the loop, we have if (k == 0 && hdl->sa_spill) followed
by an else that does a break. The result is that k != 2 will never be
evaluated when it is false. We drop the comparison.

In zap_leaf_array_read(), we have a for loop condition that is `i <
ZAP_LEAF_ARRAY_BYTES && len > 0`. However, that loop itself is in a loop
that is while (len > 0) and while the value of len is decremented
inside the loop, when len == 0, it will return, such that len > 0
inside the loop condition will always be true. We drop that part of the
condition.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14575