The explanation on how ioctl handles devid seems incorrect.  Userspace who
calls this ioctl has no input regarding devid, and ioctl implementation
retrieves devid via superblock.

Link: http://lkml.kernel.org/r/20160812024825.12352.13486.stgit@pluto.themaw.netSigned-off-by: NTomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: NIan Kent <ikent@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>