Rename callout member in struct timer_list to match the one in struct
delayed_work in the LinuxKPI. This allows the timer_pending() function
macro to be used with delayed work structures.

No functional nor structural change.

MFC after: 1 week
Submitted by: Johannes Lundberg <johalun0@gmail.com>
Sponsored by: Mellanox Technologies
Sponsored by: Limelight Networks