Fix leaking of inode memory.

Release bh in simplefs_get_inode() and use kmalloc() instead, and also
add super_operations.destory_inode handler, to free allocated memory.

This is not optimal and it must be replaced by slab allocators.
fix another inode leaking in simplefs_create_fs_object()

There we iterate over inodes using simplefs_get_inode(), while instead
we already have last inode number, just use it instead, and increment
some paddings to it (start inode, reserved inodes).

And form after this patch kedr shows that there is no leaks.
(but we must recheck it in the future.)