rootdelay on zfs should be adaptive

The 'rootdelay' boot option currently pauses the boot for a specified
amount of time. The original intent was to ensure that slower
configurations would have ample time to enumerate the devices to make
importing the root pool successful. This, however, causes unnecessary
boot delay for environments like Azure which set this parameter by
default.

This commit changes the initramfs logic to pause until it can
successfully load the 'zfs' module. The timeout specified by
'rootdelay' now becomes the maximum amount of time that initramfs will
wait before failing the boot.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Prakash Surya <prakash.surya@delphix.com>
Signed-off-by: George Wilson <gwilson@delphix.com>
Closes #14430