functionfs: fix the open/removal races

In the Linux kernel, the following vulnerability has been resolved:

functionfs: fix the open/removal races

ffs_epfile_open() can race with removal, ending up with file->private_data
pointing to freed object.

There is a total count of opened files on functionfs (both ep0 and
dynamic ones) and when it hits zero, dynamic files get removed.
Unfortunately, that removal can happen while another thread is
in ffs_epfile_open(), but has not incremented the count yet.
In that case open will succeed, leaving us with UAF on any subsequent
read() or write().

The root cause is that ffs->opened is misused; atomic_dec_and_test() vs.
atomic_add_return() is not a good idea, when object remains visible all
along.

To untangle that
* serialize openers on ffs->mutex (both for ep0 and for dynamic files)
* have dynamic ones use atomic_inc_not_zero() and fail if we had
zero ->opened; in that case the file we are opening is doomed.
* have the inodes of dynamic files marked on removal (from the
callback of simple_recursive_removal()) - clear ->i_private there.
* have open of dynamic ones verify they hadn't been already removed,
along with checking that state is FFS_ACTIVE.