mtd_blkdevs: avoid soft lockups with some mtd/spi devices

With some spi devices, the heavy cpu usage due to polling the spi
registers may lead to netdev timeouts, RCU complaints, etc. This can
be acute in the absence of CONFIG_PREEMPT. This patch allows to give
enough breathing room to avoid those incorrectly detected netdev
timeouts for example.

Example splat on 5.10.92:
[  828.399306] rcu: INFO: rcu_sched self-detected stall on CPU
...
[  828.419245] Task dump for CPU 1:
[  828.422465] task:kworker/1:1H    state:R  running task on cpu   1   stack:    0 pid:   76 ppid:     2 flags:0x0000002a
[  828.433132] Workqueue: kblockd blk_mq_run_work_fn
[  828.437820] Call trace:
...
[  828.512267]  spi_mem_exec_op+0x4d0/0xde0
[  828.516184]  spi_mem_dirmap_read+0x180/0x39c
[  828.520443]  spi_nor_read_data+0x428/0x7e8
[  828.524523]  spi_nor_read+0x154/0x214
[  828.528172]  mtd_read_oob+0x440/0x714
[  828.531815]  mtd_read+0xac/0x120
[  828.535030]  mtdblock_readsect+0x178/0x230
[  828.539102]  mtd_blktrans_work...