xfs: get our own reference to inodes that we want to scrub

xfs: get our own reference to inodes that we want to scrub

When we want to scrub a file, get our own reference to the inode
unconditionally.  This will make disposal rules simpler in the long run.

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