block: Remove redundant plug in __submit_bio()

block: Remove redundant plug in __submit_bio()

The patch removes the automatic plug/unplug operations from __submit_bio()
that were added to cache nsecs time when no explicit plug is used.

The plug mechanism is most effective when batching multiple I/O
operations together. Creating a plug for every bio submission
provides minimal benefit while adding function call overhead and
stack usage for every I/O operation.

Below is performance comparison with the latest upstream kernel.

Iotype  qd nj  rmix  mpstat busy  mpstat busy without plug
Randrw  1  20  100       53%                 24%
Randrw  1  40  100       70%                 24%
Randrw  1  20  70        40%                 24%
Randrw  1  40  70        60%                 26%
Randrw  1  20  0         14%                 6%
Randrw  1  40  0         20%                 7%

Fixes: 060406c61c7c ("block: add plug while submitting IO")
Signed-off-by: Wen Xiong <wenxiong@linux.ibm.com>
Reviewed-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260616143121.878021-1-wenxiong@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>