shiftfs: Handle multiple frees of single fs_context

The fs_context free callback can be called multiple times for the
same fs_context data, once during fsmount() and then again when
the filesystem fd is closed. Deal with this by checking that
fc->fs_private is not null before freeing private data and setting
it to NULL when private data is freed.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
1 file changed