LU-16985 utils: adaptive bufsize for mirroring

LU-16985 utils: adaptive bufsize for mirroring

if bandwidth limit is requested, then change default bufsize to
make I/O rather smooth than like a saw.

Lustre-change: https://review.whamcloud.com/51773
Lustre-commit: TBD (from 390235cc52bd51dc807492db19d8a1f905dbd9d0)

Test-Parameters: trivial
Fixes: 23224e03dc ("LU-16587 utils: give lfs migrate a larger buffer")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ibc8e7d30ded201a4ff3d699530f5c9f8be5ce7f1
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51839
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>