osc_extent_wait can be stuck in scenario like this:
1) thread-1 held an active extent
2) thread-2 called flush cache, and marked this extent as "urgent"
and "sync_wait"
3) thread-3 wants to write to the same extent, osc_extent_find will
get "conflict" because this extent is "sync_wait", so it starts
to wait...
4) cl_writeback_work has been scheduled by thread-4 to write some
other extents, it has sent RPCs but not returned yet.
5) thread-1 finished his work, and called osc_extent_release()->
osc_io_unplug_async()->ptlrpcd_queue_work(), but found
cl_writeback_work is still running, so it's ignored (-EBUSY)
6) thread-3 is stuck because nobody will wake him up.
This patch allows ptlrpcd_work to be rescheduled, so it will not
miss request anymore