Small nits

Update the comment about alloc/free ordering.

Since UEFI does probe zfs only by calling zfs probe with partition and not
with whole disk, we need zfs probe to check this case.

In D18558#396096, @alvisen_gmail.com wrote:

This broke boot on my laptop.
Single SATA SSD. UEFI boot.
loader.efi does not find my ZFS pool, which is on GPT partition 3.

=>       40  250069600  ada0  GPT  (119G)
         40       1024     1  freebsd-boot  (512K)
       1064     131072     2  efi  (64M)
     132136        984        - free -  (492K)
     133120  249935872     3  freebsd-zfs  (119G)
  250068992        648        - free -  (324K)

Small comment rerwrite

Rebase on r341328

In D18391#391262, @avg wrote:

Maybe rather than removing the code it would be better to ifdef it out?
But not sure. What are plans for illumos in this respect?

Cleaning up a bit:

Good by me - I did add this for extra safety because the malloc markers are checked on free. And since we havent been bitten yet, I guess we are good:)

In D17947#384896, @bcran wrote:

In D17947#384893, @allanjude wrote:

I think the EFI size should be 200MB or 256MB, rather than 35MB

I’m not sure that the install media should waste that much space for the ESP. It should definitely be around 200MB when being installed though.

In D17890#382275, @lev wrote:

This helps. It doen't show BSD partitions, but shows all four MBR slices without crash.

Typo fixed.

Test the presence of floppy device before attempting to read information
about it. We can get bogus information about non-existing floppy devices.

add sector size checks.

In D17131#365407, @imp wrote:

Generally I like this, but it also looks like there's some drive reset retry logic tossed in as well...

BIO_BUFFER_SIZE is defined in libi386.h

reverted comment, requested by imp.

Comments updated.

Rebase on recent rev.