ANDROID: incremental-fs: Add free options in incfs_mount

After the revert of:
"ANDROID: incremental-fs: remove index and incomplete dir on umount"
the free call was missing. Add it back to prevent a leak.

Bug: 217661925
Bug: 218732047
Bug: 219731048

Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Change-Id: I9761183720af61bee373accfb57d92ffc68a34b1