btrfs: tests: don't leak fs_info in extent_io bitmap tests

The fs_info is not freed at the end of the function and leaks. The
function is called twice so there can be up to 2x sizeof(struct
btrfs_fs_info) of leaked memory.  Fortunatelly this affects only testing
builds, the size could be 16k with several debugging features enabled.

Signed-off-by: David Sterba <dsterba@suse.com>