xfs: queue inactivation immediately when free realtime extents are tight

Now that we have made the inactivation of unlinked inodes a background
task to increase the throughput of file deletions, we need to be a
little more careful about how long of a delay we can tolerate.

Similar to the patch doing this for free space on the data device, if
the file being inactivated is a realtime file and the realtime volume is
running low on free extents, we want to run the worker ASAP so that the
realtime allocator can make better decisions.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>