common: rename struct mmap_align to mmap_align_data

common: rename struct mmap_align to mmap_align_data

In C++, g++ complains that the mmap_align free function shadows the
constructor of the structure of the same name.  Rename the structure to
mmap_align_data to avoid that.

Change-Id: Iae5e0175e3530ebc51804602240ca3037bc02c5b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7099
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>