[CHERRY-PICK] Fix FV header length (#9) (#92)

Cherry-pick from release/202202 (commit
[d484285](d484285)).

Read FV header length from the header instead of using
`sizeof(EFI_FIRMWARE_VOLUME_HEADER)` to account for variable number
of block map entries.

- [x] Impacts functionality?
- **Functionality** - Does the change ultimately impact how firmware
functions?
- Examples: Add a new library, publish a new PPI, update an algorithm,
...
- [ ] Impacts security?
- **Security** - Does the change have a direct security impact on an
application,
    flow, or firmware?
  - Examples: Crypto algorithm change, buffer overflow fix, parameter
    validation improvement, ...
- [ ] Breaking change?
- **Breaking change** - Will anyone consuming this change experience a
break
    in build or boot behavior?
- Examples: Add a new library class, move a module to a different repo,
call
    a function in a new library class in a pre-existing module, ...
- [ ] Includes tests?
  - **Tests** - Does the change include any explicit test code?
  - Examples: Unit tests, integration tests, robot tests, ...
- [ ] Includes documentation?
- **Documentation** - Does the change contain explicit documentation
additions
    outside direct code modifications (and comments)?
- Examples: Update readme file, add feature readme file, link to
documentation
    on an a separate Web page, ...

(cherry-pick from release/202202)

N/A other than including this change.

IntelSiliconPkg/Feature/Flash/SpiFvbService/FvbInfo.c

@@ -115,7 +115,7 @@ GetFvbInfo (
     Status = mFvbMediaInfoGenerators[Index](&FvbMediaInfo);
     ASSERT_EFI_ERROR (Status);
     if (!EFI_ERROR (Status) && (FvbMediaInfo.BaseAddress == FvBaseAddress)) {
-      FvHeader = AllocateCopyPool (FvbMediaInfo.FvbInfo.HeaderLength, &FvbMediaInfo.FvbInfo);
+      FvHeader = AllocateCopyPool ((FvbMediaInfo.FvbInfo.HeaderLength), &FvbMediaInfo.FvbInfo);
 
       //
       // Update the checksum value of FV header.