Call va_end() before return in zpool_standard_error_fmt()

Commit ecd6cf800b63704be73fb264c3f5b6e0dafc068d by marks in OpenSolaris
at Tue Jun 26 07:44:24 2007 -0700 introduced a bug where we fail to call
va_end() before returning.

The man page for va_start() says:

"Each invocation of va_start() must be matched by a corresponding
invocation of va_end() in the same function."

Coverity complained about this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@nutanix.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13904