mount: Fix resource leak with -V and -h

Running mount with -V or -h produced a call to fuse_mount() without
corresponding call to fuse_unmount().

This caused a small memory leak detected by valgrind.

Call fuse_unmount() to pass valgrind memcheck cleanly.

Change-Id: Ib908cbf54201938484c81f6c51fca4bb4ae93bcb