checkpatch: warn on CamelCase variable names

Store the camelcase variables in a hash and only emit a warning on the
first use of each new variable.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>