OvmfPkg/VirtioFsDxe: add helper for determining file mode bits update

OvmfPkg/VirtioFsDxe: add helper for determining file mode bits update

Add the VirtioFsGetFuseModeUpdate() function, for determining whether an
EFI_FILE_PROTOCOL.SetInfo() invocation requests an update to the file mode
bits.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3097
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201216211125.19496-48-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>