KEYS: fix refcount_inc() on zero

KEYS: fix refcount_inc() on zero

If a key's refcount is dropped to zero between key_lookup() peeking at
the refcount and subsequently attempting to increment it, refcount_inc()
will see a zero refcount.  Here, refcount_inc() will WARN_ONCE(), and
will *not* increment the refcount, which will remain zero.

Once key_lookup() drops key_serial_lock, it is possible for the key to
be freed behind our back.

This patch uses refcount_inc_not_zero() to perform the peek and increment
atomically.

Fixes: fff292914d3a2f1e ("security, keys: convert key.usage from atomic_t to refcount_t")
Signed-off-by: Mark Rutland <[email protected]>
Signed-off-by: David Howells <[email protected]>
Cc: David Windsor <[email protected]>
Cc: Elena Reshetova <[email protected]>
Cc: Hans Liljestrand <[email protected]>
Cc: James Morris <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: James Morris <[email protected]>