scsi: Avoid that toggling use_blk_mq triggers a memory leak

This patch avoids that the following memory leak is triggered if
use_blk_mq is disabled after a SCSI host has been allocated by the
ib_srp driver and before the same SCSI host is freed:

unreferenced object 0xffff8803a168c568 (size 256):
  backtrace:
    [<ffffffff81620c95>] kmemleak_alloc+0x45/0xa0
    [<ffffffff811bb104>] __kmalloc_node+0x1e4/0x400
    [<ffffffff81309fe4>] blk_mq_alloc_tag_set+0xb4/0x230
    [<ffffffff814731b7>] scsi_mq_setup_tags+0xc7/0xd0
    [<ffffffff81469c26>] scsi_add_host_with_dma+0x216/0x2d0
    [<ffffffffa064bef5>] srp_create_target+0xe55/0x13d0 [ib_srp]
    [<ffffffff8143ce23>] dev_attr_store+0x13/0x20
    [<ffffffff8125f030>] sysfs_kf_write+0x40/0x50
    [<ffffffff8125e397>] kernfs_fop_write+0x137/0x1c0
    [<ffffffff811d8c13>] __vfs_write+0x23/0x140
    [<ffffffff811d92e0>] vfs_write+0xb0/0x190
    [<ffffffff811da5b4>] SyS_write+0x44/0xa0
    [<ffffffff8162c8a5>] entry_SYSCALL_64_fastpath+0x18/0xa8

Fixes: 9aa9cc42 ("scsi: remove the disable_blk_mq host flag")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>