libzfs: Add error message for why creating mountpoint failed

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 <[email protected]>
Signed-off-by: Ryan Moeller <[email protected]>
Closes #10550 

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