include/linux/zutil.h: fix usage example of zlib_adler32()

alder32 was renamed to zlib_adler32 since before 2.6.11.

Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>