->pre_attach() is supposed to be called before migration, which is
observed during process migration but task migration does it the other
way around.  The only ->pre_attach() user is cpuset which can do the
same operaitons in ->can_attach().  Collapse cpuset_pre_attach() into
cpuset_can_attach().

-v2: Patch contamination from later patch removed.  Spotted by Paul
     Menage.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reviewed-by: NFrederic Weisbecker <fweisbec@gmail.com>
Acked-by: NPaul Menage <paul@paulmenage.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>