Fix vn_open/vn_rdwr error handling

1. In vn_open(), if fstat64() returned an error, the real errno

was being obscured by calling close().

2. Add error handling for both pwrite64() calls in vn_rdwr().

Signed-off-by: Ricardo M. Correia <Ricardo.M.Correia@Sun.COM>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>