checkpatch: fix left brace warning

Using checkpatch.pl with Perl 5.22.0 generates the following warning:

    Unescaped left brace in regex is deprecated, passed through in regex;

This patch fixes the warnings by escaping occurrences of the left brace
inside the regular expression.

Signed-off-by: Eddie Kovsky <ewk@edkovsky.org>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>