The merge omitted some InnoDB and XtraDB conflict resolutions,
most notably, failing to merge the fix of MDEV-12173.

ibuf_merge_or_delete_for_page(), lock_rec_block_validate():
Invoke fil_space_acquire_silent() instead of fil_space_acquire().
This fixes MDEV-12173.

wsrep_debug, wsrep_trx_is_aborting(): Removed unused declarations.

_fil_io(): Remove. Instead, declare default parameters for the XtraDB
fil_io().

buf_read_page_low(): Declare default parameters, and clean up some
callers.

os_aio(): Correct the macro that is defined when !UNIV_PFS_IO.