ibuf_inside(), ibuf_enter(), ibuf_exit(): Add the parameter mtr. The
flag is no longer kept in the thread-local storage but in the
mini-transaction (mtr->inside_ibuf).

mtr_start(): Clean up the comment and remove the unused return value.
mtr_commit(): Assert !ibuf_inside(mtr) in debug builds.

ibuf_mtr_start(): Like mtr_start(), but sets the flag.
ibuf_mtr_commit(), ibuf_btr_pcur_commit_specify_mtr(): Wrappers that
assert ibuf_inside().

buf_page_get_zip(), buf_page_init_for_read(),
buf_read_ibuf_merge_pages(), fil_io(), ibuf_free_excess_pages(),
ibuf_contract_ext(): Remove assertions on ibuf_inside(), because a
mini-transaction is not available.

buf_read_ahead_linear(): Add the parameter inside_ibuf.

ibuf_restore_pos(): When this function returns FALSE, it commits mtr
and must therefore do ibuf_exit(mtr).

ibuf_delete_rec(): This function commits mtr and must therefore do
ibuf_exit(mtr).

ibuf_rec_get_page_no(), ibuf_rec_get_space(), ibuf_rec_get_info(),
ibuf_rec_get_op_type(), ibuf_build_entry_from_ibuf_rec(),
ibuf_rec_get_volume(), ibuf_get_merge_page_nos(),
ibuf_get_volume_buffered_count(), ibuf_get_entry_counter_low(): Add
the parameter mtr in debug builds, for asserting ibuf_inside(mtr).

rb:585 approved by Sunny Bains