xfs: remove XFS_FIND_RCEXT_SHARED and _COW

Now that we have an explicit enum for shared and CoW staging extents, we
can get rid of the old FIND_RCEXT flags.  Omit a couple of conversions
that disappear in the next patches.

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