LU-13031 jobstats: store jobid in xattr when files are created

LU-13031 jobstats: store jobid in xattr when files are created

This change stores the jobid of the process that creates a file in an
extended attribute in the file's MDT inode, at file creation time.

The name of the xattr is determined by a new sysfs parameter
"mdt.*.job_xattr" so that the admin can choose a name that does
not conflict with other uses they may have for a given xattr.
The default value is "user.job". A value of "NONE" means that
the jobid will not be stored in the inode.

If the name is in the user namespace "user.", then the name portion
can be up to 7 alphanumeric characters long. The admin can choose
the trusted namespace to prevent users from modifying the value,
but only "trusted.job" is allowed in this namespace.

Allowing users to modify the contents of the xattr is helpful so
that the jobid can be preserved even when files are moved with tools
like `cp` or `rsync`, and when copied from one filesystem to another.

Signed-off-by: Thomas Bertschinger <bertschinger@lanl.gov>
Change-Id: Iad78a5ec6fbc4b761ff481141763bdd0cdcd0128
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50982
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>