PM / Sleep: Fix read_unlock_usermodehelper() call.

Commit b298d289
 "PM / Sleep: Fix freezer failures due to racy usermodehelper_is_disabled()"
added read_unlock_usermodehelper() but read_unlock_usermodehelper() is called
without read_lock_usermodehelper() when kmalloc() failed.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>