illumos gate

ZoL: 0c637f3100f zpool reports 16E expandsize on disks with oddball number of sector

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().