OvmfPkg/VirtioFsDxe: suppress incorrect gcc warnings

OvmfPkg/VirtioFsDxe: suppress incorrect gcc warnings

`CreateDirectoryIfCreating` is used only if `PermitCreation` is set.

`NewNodeIsDirectory` might not set in case of error, but that would lead
to leaving the function before invalid use.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3228
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Sergei Dmitrouk <sergei@posteo.net>
Message-Id: <20210511225616.5942-3-sergei@posteo.net>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>