LU-16587 utils: give lfs migrate a larger buffer

LU-16587 utils: give lfs migrate a larger buffer

lfs migrate is slow because it mostly uses a small 1MB buffer. Bigify.

[root@kjlmo4n00 16G]# time lfs migrate -S 1M -p flash 16G.1
real	0m25.341s
[root@kjlmo4n00 16G]# time /root/tools/lfs_nzr migrate -S 1M -p flash 16G.1
real	0m6.526s

Signed-off-by: Nathan Rutman <nathan.rutman@hpe.com>
Change-Id: I850ca475fcd0efe2d71d26e4d1544f462c60252a
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50118
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>