rmap: make anon_vma_chain_free() static

Make anon_vma_chain_free() static.  It is called only in rmap.c and the
corresponding alloc function is already static.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>