checkpatch: avoid suggesting struct definitions should be const

Many structs are generally used const and there is a known list of these
structs.

struct definitions should not be generally be declared const.

Add a test for the lack of an open brace immediately after the struct to
avoid definitions.

This avoids the false positive "struct foo should normally be const"
message only when the open brace is on the same line as the definition.

Link: http://lkml.kernel.org/r/0dce709150d712e66f1b90b03827634b53b28085.1491845946.git.joe@perches.com


Signed-off-by: Joe Perches <joe@perches.com>
Cc: Arthur Brainville <ybalrid@ybalrid.info>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>