kmod: introduce call_modprobe() helper

kmod: introduce call_modprobe() helper

BugLink: http://bugs.launchpad.net/bugs/963685

No functional changes.  Move the call_usermodehelper code from
__request_module() into the new simple helper, call_modprobe().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Tejun Heo <tj@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 3e63a93b987685f02421e18b2aa452d20553a88b)

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>