checkpatch: improve OPEN_BRACE test

checkpatch: improve OPEN_BRACE test

Some structure definitions that use macros trip the OPEN_BRACE test.

e.g. +struct bpf_map_def SEC("maps") control_map = {

Improve the test by using $balanced_parens instead of a .*

Miscellanea:

o Use $sline so any comments are ignored
o Correct the message output from declaration to definition
o Remove unnecessary parentheses

Link: http://lkml.kernel.org/r/db9b772999d1d2fbda3b9ee24bbca81a87837e13.1517543491.git.joe@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>