Rename hdr_len to ext_len

'hdr_len' is actually the length of the extras, so give it a name
which reflects that

Change-Id: Id0f6d0c659b1cfa594dcf50f965955282086e5b8
Reviewed-on: http://review.couchbase.org/44712
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: buildbot <build@couchbase.com>