kobj_read_file: Return -1 on vn_rdwr() error

LLVM's static analyzer showed that we could subtract using an
uninitialized value on an error from vn_rdwr().

The correct behavior is to return -1 on an error, so lets do that
instead.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4104