xfs_scrub: implement deferred description string rendering

xfs_scrub: implement deferred description string rendering

A flamegraph analysis of xfs_scrub runtimes showed that we spend 7-10%
of the program's userspace runtime rendering prefix strings in case we
want to show a message about something we're checking, whether or not
that string ever actually gets used.

For a non-verbose run on a clean filesystem, this work is totally
unnecessary.  We could defer the message catalog lookup and snprintf
call until we actually need that message, so build enough of a function
closure mechanism so that we can capture some location information when
its convenient and push that all the way to the edge of the call graph
and only when we need it.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
[sandeen: make comment change suggested on list]
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>