populate: improve attr creation runtime

populate: improve attr creation runtime

Replace the file creation loops with a python script that does
everything we want from a single process.  This reduces the runtime of
_scratch_xfs_populate substantially by avoiding thousands of execve
overhead.  This patch builds on the previous one by reducing the runtime
of xfs/349 from ~45s to ~15s.

For people who don't have python3, use setfattr's "restore" mode to bulk
create xattrs.  This reduces runtime to about ~25s.

[zlang: add popattr.py into src/Makefile install list]

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>