KEYS: Fix __key_link_end() quota fixup on error

Fix __key_link_end()'s attempt to fix up the quota if an error occurs.

There are two erroneous cases: Firstly, we always decrease the quota if
the preallocated replacement keyring needs cleaning up, irrespective of
whether or not we should (we may have replaced a pointer rather than
adding another pointer).

Secondly, we never clean up the quota if we added a pointer without the
keyring storage being extended (we allocate multiple pointers at a time,
even if we're not going to use them all immediately).

We handle this by setting the bottom bit of the preallocation pointer in
__key_link_begin() to indicate that the quota needs fixing up, which is
then passed to __key_link() (which clears the whole thing) and
__key_link_end().

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>