mkfs: allow sizing realtime allocation groups for concurrency

mkfs: allow sizing realtime allocation groups for concurrency

Add a -r concurrency= option to mkfs so that sysadmins can configure the
filesystem so that there are enough rtgroups that the specified number
of threads can (in theory) can find an uncontended rtgroup from which to
allocate space.  This has the exact same purpose as the -d concurrency
switch that was added for the data device.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>