virtio_fs: introduce virtio_fs_put_locked helper

Introduce a new helper function virtio_fs_put_locked to encapsulate the
common pattern of releasing a virtio_fs reference while holding a lock.
The existing virtio_fs_put helper will be used to release a virtio_fs
reference while not holding a lock.

Also add an assertion in case the lock is not taken when it should.

Reviewed-by: Idan Zach <izach@nvidia.com>
Reviewed-by: Shai Malin <smalin@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Message-Id: <20240825130716.9506-1-mgurtovoy@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>