LU-19119 misc: add umd_ prefix for fields in lnet_md

LU-19119 misc: add umd_ prefix for fields in lnet_md

All structure fields should have a prefix to make the study of code
easier. It was not the case for struct lnet_md.
Some cosmetic changes have also been made.

Test-Parameters: trivial
Signed-off-by: Cyril Bordage <cbordage@whamcloud.com>
Change-Id: I725be4f5ebac3537e64197a648ea4261a710c37b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59865
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>