[PATCH] kref: avoid an atomic operation in kref_put()

Avoid an atomic operation in kref_put() when the last reference is
dropped. On most platforms, atomic_read() is a plan read of the counter
and involves no atomic at all.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>