vdev_disk_error() prints ASCII SOH to debug log

vdev_disk_error() prints ASCII SOH to debug log

Currently vdev_disk_error() prepends its messages sent to the internal
ZFS debug log with KERN_WARNING, which is currently defined as follows:

   #define KERN_SOH      "\001"
   #define KERN_WARNING  KERN_SOH "4"

Since "\001" (ASCII Start Of Header) is not printable this results in
weird characters displayed when inspecting the debug log. This commit
simply removes this superfluous prefix passed to zfs_dbgmsg().

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7936