vfio/pci: Fix potential memory leak in vfio_msi_cap_len

[ Upstream commit 30ea32ab1951c80c6113f300fce2c70cd12659e4 ]

Free allocated vdev->msi_perm in error path.

Signed-off-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>