populate: remove file creation loops that take forever

populate: remove file creation loops that take forever

Replace the file creation loops with a perl 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.  On my system, this reduces the runtime of xfs/349 (with scrub
enabled) from ~140s to ~45s.

[zlang: add popdir.pl 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>