Fill in mountpoint buffer before using it in errors

zfs_is_mountable() fills in the mountpoint buffer, so, as in
upstream, it needs to have been called before the mountpoint
buffer can be used in error messages.

In particular,

	return (zfs_error_fmt(hdl, EZFS_MOUNTFAILED,
	    dgettext(TEXT_DOMAIN, "cannot mount '%s'"),
	    mountpoint));

should not come before the call to zfs_is_mountable().