storage: avoid memory leak

storage: avoid memory leak

Coverity detected that options was being set by strdup but never
freed.  But why even bother with an options variable?  The options
parameter never changes!  Leak present since commit 44948f5b (0.7.0).

This function could probably be rewritten to take better advantage
of virCommand, but that is more invasive.

* src/storage/storage_backend_fs.c
(virStorageBackendFileSystemMount): Avoid wasted strdup, and
guarantee proper cleanup on all paths.