checkpatch: improve the embedded function name test for patch contexts

The current test works only for a single patch context as it is done in
the foreach ($rawlines) loop that precedes the loop where the actual
$context_function variable is used.

Move the set of $context_function into the foreach (@lines) loop where
it is useful for each patch context.

Link: http://lkml.kernel.org/r/6c675a31c74fbfad4fc45b9f462303d60ca2a283.1493486091.git.joe@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>