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>