Fix gcc fortify source warnings

Resolve issues uncovered by -D_FORTIFY_SOURCE=2, the default redhat
macro's file adds this option to the cflags.  This causes warnings
of the following type designed to keep the developer honest:

        warning: ignoring return value of 'foo', declared
                 with attribute warn_unused_result

The short term fix is to wrap these calls in VERIFY() to check the
return code.  The code was already assusing these would never fail.

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

lib/libzpool/kernel.c

@@ -796,7 +796,7 @@ umem_out_of_memory(void)
 {
 	char errmsg[] = "out of memory -- generating core dump\n";
 
-	write(fileno(stderr), errmsg, sizeof (errmsg));
+	(void) fprintf(stderr, "%s", errmsg);
 	abort();
 	return (0);
 }