kref: Implement kref_get_unless_zero v3

kref: Implement kref_get_unless_zero v3

commit 4b20db3de8dab005b07c74161cb041db8c5ff3a7 upstream.

This function is intended to simplify locking around refcounting for
objects that can be looked up from a lookup structure, and which are
removed from that lookup structure in the object destructor.
Operations on such objects require at least a read lock around
lookup + kref_get, and a write lock around kref_put + remove from lookup
structure. Furthermore, RCU implementations become extremely tricky.
With a lookup followed by a kref_get_unless_zero *with return value check*
locking in the kref_put path can be deferred to the actual removal from
the lookup structure and RCU lookups become trivial.

v2: Formatting fixes.
v3: Invert the return value.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
[bwh: Backported to 3.2:
 - Adjust context
 - Add #include <linux/atomic.h>]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>