zfs_dbgmsg() is not safe from every context

zfs_dbgmsg() is not safe from every context

This commit reverts to using printk() instead of zfs_dbgmsg() to log
messages in vdev_disk_error(): this is necessary because the latter can
be called from interrupt context where we are not allowed to sleep.
Unfortunately zfs_dbgmsg() performs its allocations calling kmalloc()
with the KM_SLEEP flag which may result in the following oops:

   BUG: scheduling while atomic: swapper/4/0/0x10000100
	Call Trace:
	<IRQ>  [<0>] dump_stack+0x19/0x1b
	...
	[<0>] spl_kmem_alloc+0xdf/0x140 [spl] <-- kmem_alloc(size, KM_SLEEP)
	[<0>] __dprintf+0x69/0x150 [zfs]
	[<0>] ? kmem_cache_free+0x1e2/0x200
	[<0>] vdev_disk_error.part.15+0x5f/0x70 [zfs]
	[<0>] vdev_disk_io_flush_completion+0x48/0x70 [zfs]
	[<0>] bio_endio+0x67/0xb0
	[<0>] blk_update_request+0x90/0x360
	...
	[<0>] scsi_finish_command+0xdc/0x140
	[<0>] scsi_softirq_done+0x132/0x160
	[<0>] blk_done_softirq+0x96/0xc0
	[<0>] __do_softirq+0xf5/0x280
	[<0>] call_softirq+0x1c/0x30
	[<0>] do_softirq+0x65/0xa0
	[<0>] irq_exit+0x105/0x110
	[<0>] do_IRQ+0x56/0xf0
	[<0>] common_interrupt+0x162/0x162
	<EOI>  [<0>] ? cpuidle_enter_state+0x54/0xd0
	[<0>] cpuidle_idle_call+0xde/0x230
	[<0>] arch_cpu_idle+0xe/0xb0
	[<0>] cpu_startup_entry+0x14a/0x1e0
	[<0>] start_secondary+0x1f7/0x270
	[<0>] start_cpu+0x5/0x14

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8137
Closes #8150