fh_to_dentry should return ESTALE when generation mismatch

When generation mismatch, it usually means the file pointed by the file handle
was deleted. We should return ESTALE to indicate this. We return ENOENT in
zfs_vget since zpl_fh_to_dentry will convert it to ESTALE.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #4828