A caller initiates the drain procces from its context once the
drain threshold is reached or passed. There are at least two
drawbacks of doing so:

a) a caller can be a high-prio or RT task. In that case it can
   stuck in doing the actual drain of all lazily freed areas.
   This is not optimal because such tasks usually are latency
   sensitive where the control should be returned back as soon
   as possible in order to drive such workloads in time. See
   96e2db45 ("mm/vmalloc: rework the drain logic")

b) It is not safe to call vfree() during holding a spinlock due
   to the vmap_purge_lock mutex. The was a report about this from
   Zeal Robot <zealci@zte.com.cn> here:
   https://lore.kernel.org/all/20211222081026.484058-1-chi.minghao@zte.com.cn

Moving the drain to the separate work context addresses those
issues.

v1->v2:
   - Added prefix "_work" to the drain worker function.
v2->v3:
   - Remove the drain_vmap_work_in_progress. Extra queuing
     is expectable under heavy load but it can be disregarded
     because a work will bail out if nothing to be done.

Link: https://lkml.kernel.org/r/20220131144058.35608-1-urezki@gmail.comSigned-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Cc: Uladzislau Rezki <uladzislau.rezki@sony.com>
Cc: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>