zpool reports 16E expandsize on disks with oddball number of sectors

The issue is caused by a small discrepancy in how userland creates the
partition layout and the kernel estimates available space:

 * zpool command: subtract 9M from the usable device size, then align
   to 1M boundary. 9M is the sum of 1M "start" partition alignment + 8M
   EFI "reserved" partition.

 * kernel module: subtract 10M from the device size. 10M is the sum of
   1M "start" partition alignment + 1m "end" partition alignment + 8M
   EFI "reserved" partition.

For devices where the number of sectors is not a multiple of the
alignment size the zpool command will create a partition layout which
reserves less than 1M after the 8M EFI "reserved" partition:

  Disk /dev/sda: 1024 MiB, 1073739776 bytes, 2097148 sectors
  Units: sectors of 1 * 512 = 512 bytes
  Sector size (logical/physical): 512 bytes / 512 bytes
  I/O size (minimum/optimal): 512 bytes / 512 bytes
  Disklabel type: gpt
  Disk identifier: 49811D40-16F4-4E41-84A9-387703950D7F

  Device       Start     End Sectors  Size Type
  /dev/sda1     2048 2078719 2076672 1014M Solaris /usr & Apple ZFS
  /dev/sda9  2078720 2095103   16384    8M Solaris reserved 1

When the kernel module vdev_open() the device its max_asize ends up
being slightly smaller than asize: this results in a huge number (16E)
reported by metaslab_class_expandable_space().

This change prevents bdev_max_capacity() from returing a size smaller
than bdev_capacity().

Signed-off-by: loli10K <ezomori.nozomu@gmail.com>

module/zfs/vdev_disk.c

@@ -126,10 +126,7 @@ bdev_max_capacity(struct block_device *bdev, uint64_t wholedisk)
 		available = i_size_read(bdev->bd_contains->bd_inode) -
 		    ((EFI_MIN_RESV_SIZE + NEW_START_BLOCK +
 		    PARTITION_END_ALIGNMENT) << SECTOR_BITS);
-		if (available > 0)
-			psize = available;
-		else
-			psize = bdev_capacity(bdev);
+		psize = MAX(available, bdev_capacity(bdev));
 	} else {
 		psize = bdev_capacity(bdev);
 	}