When zfs_mount_at() fails to stat the mountpoint and can't create the
directory, we return an error with a message "failed to create
mountpoint" but there is no indication why it failed.
Add the error string from the syscall to the error aux message.
Update do_mount for Linux to return the errno instead of -1.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes#10550
217f48373f