9pfs: clarify latency of v9fs_co_run_in_worker()

9pfs: clarify latency of v9fs_co_run_in_worker()

As we just fixed a severe performance issue with Treaddir request
handling, clarify this overall issue as a comment on
v9fs_co_run_in_worker() with the intention to hopefully prevent
such performance mistakes in future (and fixing other yet
outstanding ones).

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <4d34d332e1aaa8a2cf8dc0b5da4fd7727f2a86e8.1596012787.git.qemu_oss@crudebyte.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>