char_dev: extend dynamic allocation of majors into a higher range

We've run into problems with running out of dynamicly assign char
device majors particullarly on automated test systems with
all-yes-configs. Roughly 40 dynamic assignments can be made with such
kernels at this time while space is reserved for only 20.

Currently, the kernel only prints a warning when dynamic allocation
overflows the reserved region. And when this happens drivers that have
fixed assignments can randomly fail depending on the order of
initialization of other drivers. Thus, adding a new char device can cause
unexpected failures in completely unrelated parts of the kernel.

This patch solves the problem by extending dynamic major number
allocations down from 511 once the 234-254 region fills up. Fixed
majors already exist above 255 so the infrastructure to support
high number majors is already in place. The patch reserves an
additional 128 major numbers which should hopefully last us a while.

Kernels that don't require more than 20 dynamic majors assigned (which
is pretty typical) should not be affected by this change.

Signed-off-by: Logan Gunthorpe <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Alan Cox <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Linus Walleij <[email protected]>
Link: https://lkml.org/lkml/2017/6/4/107
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Documentation/admin-guide/devices.txt

@@ -3081,3 +3081,8 @@
 		  1 = /dev/osd1		Second OSD Device
 		  ...
 		  255 = /dev/osd255	256th OSD Device
+
+ 384-511 char	RESERVED FOR DYNAMIC ASSIGNMENT
+		Character devices that request a dynamic allocation of major
+		number will take numbers starting from 511 and downward,
+		once the 234-254 range is full.