percpu_ref: Fix comment regarding percpu_ref_init flags

The comment for percpu_ref_init() implies that using
PERCPU_REF_ALLOW_REINIT will cause the refcount to start at 0.  But
this is not true.  PERCPU_REF_ALLOW_REINIT starts the count at 1 as
if the flags were zero.  Add this fact to the kernel doc comment.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
[Dennis: reworded]
Signed-off-by: Dennis Zhou <dennis@kernel.org>