buf_dump(): Correct the printf format passed to buf_dump_status()
to match the argument types.

Revert the changes to storage/xtradb. XtraDB is not being compiled
for 10.2. The unused copy that we have in the 10.2 branch is only
getting merges from 10.1.

Disable the test sys_vars.innodb_buffer_pool_dump_pct_function
because it is unstable on buildbot.