FreeBSD: Use ashift in vdev_check_boot_reserve()

[amotin]

We should not hardcode 512-byte read size when checking for loader in the boot area before RAIDZ expansion. Disk might be unable to handle that I/O as is, and the code zio_vdev_io_start() handling the padding asserts doing it only for top-level vdev.

How Has This Been Tested?

Create a raidz pool with ashift > 9, try to expand it and observe the assertion panic. Apply the patch and see it working.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.