flex_array: fix flex_array_free_parts comment

flex_array_free_parts() does not take `src' or `element_nr' formals, so
remove their respective comments.

Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>