Fix device expansion when VM is powered off

Fix device expansion when VM is powered off

When running on an ESXi based VM, I've found that "zpool online -e" will
not expand the zpool, if the disk was expanded in ESXi while the VM was
powered off.

For example, take the following scenario:

 1. VM running on top of VMware ESXi
 2. ZFS pool created with a given device "sda" of size 8GB
 3. VM powered off
 4. Device "sda" size expanded to 16GB
 5. VM powered on
 6. "zpool online -e" used on device "sda"

In this situation, after (2) the zpool will be roughly 8GB in size.
After (6), the expectation is the zpool's size will expand to roughly
16GB in size; i.e. expand to the new size of the "sda" device.
Unfortunately, I've seen that after (6), the zpool size does not change.

What's happening is after (5), the EFI label of the "sda" device will be
such that fields "efi_last_u_lba", "efi_last_lba", and "efi_altern_lba"
all reflect the new size of the disk; i.e. "33554398", "33554431", and
"33554431" respectively.

Thus, the check that we perform in "efi_use_whole_disk":

    if ((efi_label->efi_altern_lba == 1) || (efi_label->efi_altern_lba
        >= efi_label->efi_last_lba)) {

This will return true, and then we return from the function without
having expanded the size of the zpool/device.

In contrast, if we remove steps (3) and (5) in the sequence above, i.e.
the device is expanded while the VM is powered on, things change. In
that case, the fields "efi_last_u_lba" and "efi_altern_lba" do not
change (i.e. they still reflect the old 8GB device size), but the
"efi_last_lba" field does change (i.e. it now reflects the new 16GB
device size). Thus, when we evaluate the same conditional in
"efi_use_whole_disk", it'll return false, so the zpool is expanded.

Taking all of this into account, this PR updates "efi_use_whole_disk" to
properly expand the zpool when the underlying disk is expanded while the
VM is powered off.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Don Brady <don.brady@delphix.com>
Signed-off-by: Prakash Surya <prakash.surya@delphix.com>
Closes #9111