LU-15367 llite: iotrace standardization

LU-15367 llite: iotrace standardization

Clean up and standardize some of the iotrace messages for
easier parsing.

Add a clear 'START' indicator.

Remove a now-redundant debug message in the mmap code.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ia620cc8c783509cbc3f47b21a274d67d860b80e7
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52002
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>