target: remove some smp_mb__after_atomic()s

atomic_inc_return() already does an implicit memory barrier and the
second case was moved from an atomic to a plain flag operation.  If a
barrier were needed in the second case, it would have to be smp_mb(),
not a variant optimized away for x86 and other architectures.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>