[Bug 211028] [GEOM][Hyper-V] gpart can't detect the new free space after the disk capacity changes

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=211028

            Bug ID: 211028
           Summary: [GEOM][Hyper-V] gpart can't detect the new free space
                    after the disk capacity changes
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: decui at microsoft.com

When testing Hyper-V's Online VHDX Resize feature (refer to Bug 211000), I
noticed an issue: after resizing “da1”, if there was a disk read before
"camcontrol reprobe da1", gpart couldn't detect the new “free space", though it
could detect the new disk capacity.

To reproduce the bug, we need to run FreeBSD 12-CURRENT guest on Hyper-V (we
need to make sure the 2 patches mentioned by
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=211000#c4 are used to avoid
the known issues) and do the following

1. This is before the disk capacity change:
[root at decui-b11 ~]# gpart show  da1
=>      63  83886017  da1  MBR  (40G)
        63  83886017       - free -  (40G)

2. Expand the disk capacity from 40GB to 50GB by Hyper-V management tool.

3. 
[root at decui-b11 ~]# dd if=/dev/da1 of=/dev/null bs=512 count=1
1+0 records in
1+0 records out
512 bytes transferred in 0.000715 secs (715864 bytes/sec)
[root at decui-b11 ~]# gpart show  da1
=>      63  83886017  da1  MBR  (50G)
        63  83886017       - free -  (40G)

We can see the 'free' space can't grow to 50GB.

4. This can't help:
[root at decui-b11 ~]# camcontrol reprobe da1
[root at decui-b11 ~]# gpart show  da1
=>      63  83886017  da1  MBR  (50G)
        63  83886017       - free -  (40G)

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