xfs: flush inodegc before swapon

Fix the brand new xfstest that tries to swapon on a recently unshared
file and use the chance to document the other bit of magic in this
function.

The big comment is taken from a mailinglist post by Dave Chinner.

Fixes: 5e672cd6 ("xfs: introduce xfs_inodegc_push()")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>