xfs: flush inode inactivation work when compiling usage statistics

Users have come to expect that the space accounting information in
statfs and getquota reports are fairly accurate.  Now that we inactivate
inodes from a background queue, these numbers can be thrown off by
whatever resources are singly-owned by the inodes in the queue.  Flush
the pending inactivations when userspace asks for a space usage report.

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